Load-sharing bone anchor having a deflectable post with a compliant ring and method for stabilization of the spine

ABSTRACT

A dynamic bone anchor for anchoring a spine stabilization assembly which supports the spine while providing for the preservation of spinal motion. The dynamic bone anchor provides load sharing while preserving range of motion and reducing stress exerted upon the bone anchors and spinal anatomy. The dynamic bone anchor includes a deflectable post connected by a ball-joint to a threaded anchor. Deflection of the deflectable post is controlled by a compliant ring. The force/deflection properties of the dynamic bone anchor may be adapted to the anatomy and functional requirements of the patient. The dynamic bone anchor may be used as a component of a dynamic stabilization system which supports the spine while providing for the preservation of spinal motion.

CLAIM TO PRIORITY

This application claims priority to the following patents and patentapplications:

U.S. Provisional Application No. 61/100,593 filed Sep. 26, 2008,entitled “A Spine Implant With A Deflection Rod System SelectivelyAlignable And Selectively Lockable To A Bone Anchor And Method” ; and

U.S. Provisional Application No. 61/100,625 filed Sep. 26, 2008,entitled “Versatile Components And Methods For Dynamic Stabilization” ;and

U.S. Provisional Application No. 61/119,651 filed Dec. 3, 2008, entitled“Load-sharing Component Having A Deflectable Post And Methods ForDynamic Spinal Stabilization” ; and

U.S. Provisional Application No. 61/122,658 filed Dec. 15, 2008,entitled “Load-sharing Component Having A Deflectable Post And MethodsFor Dynamic Spinal Stabilization” ; and

U.S. Provisional Application No. 61/144,426 filed Jan. 13, 2009,entitled “Load-sharing Component Having A Deflectable Post And MethodsFor Dynamic Spinal Stabilization” ; and

U.S. Provisional Application No. 61/225,478 filed Jul. 14, 2009,entitled “Load-sharing Component Having A Deflectable Post And MethodsFor Dynamic Spinal Stabilization” ; and

U.S. Provisional Application No. 61/167,789 filed Apr. 8, 2009, entitled“Load-sharing Component Having A Deflectable Post And Spring And MethodsFor Dynamic Spinal Stabilization” ; and

U.S. Provisional Application No. 61/217,556 filed Jun. 1, 2009, entitled“Load-sharing Component Having A Deflectable Post AndAxially-Compressible Spring And Methods For Dynamic SpinalStabilization” .

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/130,395, filed May 30, 2008, entitled “ADeflection Rod System For A Dynamic Stabilization And MotionPreservation Spinal Implantation System And Method” which claimspriority to U.S. Provisional Application No. 61/031,598 filed Feb. 26,2008 and entitled “A Deflection Rod System For A Dynamic StabilizationAnd Motion Preservation Spinal Implantation System And Method”.

The present application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/130,095, filed May 30, 2008, entitled “A SpineImplant With A Deflection Rod System Including A Deflection LimitingShield Associated With A Bone Screw And Method” which claims priority toU.S. Provisional Application No. 61/057,340 filed May 30, 2008, entitled“A Spine Implant With A Deflection Rod System Aligned With A Bone AnchorAnd Method” .

All of the afore-mentioned patent applications are incorporated hereinby reference in their entireties.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to all of the afore-mentioned patentapplications. This application is also related to all of the followingapplications including:

U.S. patent application Ser. No. 12/566,478, filed Sep. 24, 2009,entitled “A Modular In-Line Deflection Rod And Bone Anchor System AndMethod For Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,485, filed Sep. 24, 2009,entitled “Versatile Polyaxial Connector Assembly And Method For DynamicStabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,487, filed Sep. 24, 2009,entitled “Versatile Offset Polyaxial Connector And Method For DynamicStabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,491, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post and MethodFor Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,494, filed Sep. 24, 2009,entitled “Load-Sharing Component Having A Deflectable Post And MethodFor Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,498, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Durable Compliant Member AndMethod For Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,504, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post With ACompliant Ring And Method For Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,511, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post And MethodFor Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,516, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post And MethodFor Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,519, filed Sep. 24, 2009,entitled “Dynamic Spinal Rod And Method For Dynamic Stabilization Of TheSpine” . SPART-01044USC); and

U.S. patent application Ser. No. 12/566,522, filed Sep. 24, 2009,entitled “Dynamic Spinal Rod Assembly And Method For DynamicStabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,529, filed Sep. 24, 2009,entitled “Configurable Dynamic Spinal Rod And Method For DynamicStabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,531, filed Sep. 24, 2009,entitled “A Spinal Prosthesis Having A Three Bar Linkage For MotionPreservation And Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,534, filed Sep. 24, 2009,entitled “Surgical Tool And Method For Implantation of A Dynamic BoneAnchor” ; and

U.S. patent application Ser. No. 12/566,547, filed Sep. 24, 2009,entitled “Surgical Tool And Method For Connecting A Dynamic Bone Anchorand Dynamic Vertical Rod” ; and

U.S. patent application Ser. No. 12/566,551, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post AndCentering Spring And Method For Dynamic Stabilization Of The Spine” ;and

U.S. patent application Ser. No. 12/566,553, filed Sep. 24, 2009,entitled “Load-Sharing Component Having A Deflectable Post And CenteringSpring And Method For Dynamic Stabilization Of The Spine” ; and

U.S. patent application Ser. No. 12/566,559, filed Sep. 24, 2009,entitled “Load-Sharing Bone Anchor Having A Deflectable Post And AxialSpring And Method For Dynamic Stabilization Of The Spine” .

All of the afore-mentioned patent applications are incorporated hereinby reference in their entireties.

BACKGROUND OF INVENTION

Back pain is a significant clinical problem and the costs to treat it,both surgical and medical, are estimated to be over $2 billion per year.One method for treating a broad range of degenerative spinal disordersis spinal fusion. Implantable medical devices designed to fuse vertebraeof the spine to treat have developed rapidly over the last decade.However, spinal fusion has several disadvantages including reduced rangeof motion and accelerated degenerative changes adjacent the fusedvertebrae.

Alternative devices and treatments have been developed for treatingdegenerative spinal disorders while preserving motion. These devices andtreatments offer the possibility of treating degenerative spinaldisorders without the disadvantages of spinal fusion. However, currentdevices and treatments suffer from disadvantages e.g., complicatedimplantation procedures; lack of flexibility to conform to diversepatient anatomy; the need to remove tissue and bone for implantation;increased stress on spinal anatomy; insecure anchor systems; poordurability, and poor revision options. Consequently, there is a need fornew and improved devices and methods for treating degenerative spinaldisorders while preserving motion.

SUMMARY OF INVENTION

The present invention includes a spinal implant system and methods thatcan dynamically stabilize the spine while providing for the preservationof spinal motion. Embodiments of the invention provide a dynamicstabilization system which includes: versatile components, adaptablestabilization assemblies, and methods of implantation. An aspect ofembodiments of the invention is the ability to stabilize two, threeand/or more levels of the spine by the selection of appropriatecomponents of embodiments of the invention for implantation in apatient. Another aspect of embodiments of the invention is the abilityto accommodate particular anatomy of the patient by providing a systemof versatile components which may be customized to the anatomy and needsof a particular patient and procedure. Another aspect of the inventionis to facilitate the process of implantation and minimize disruption oftissues during implantation.

Thus, the present invention provides new and improved systems, devicesand methods for treating degenerative spinal disorders by providing andimplanting a dynamic spinal stabilization assembly which supports thespine while preserving motion. These and other objects, features andadvantages of the invention will be apparent from the drawings anddetailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective views of a deflection system componentmounted to an anchor system component according to an embodiment of thepresent invention.

FIG. 1C is a perspective view of a connection system component mountedto an anchor system component according to an embodiment of the presentinvention.

FIG. 1D is a perspective view of a different connection system componentmounted to an anchor system component according to an embodiment of thepresent invention.

FIG. 1E is a posterior view of an anchor system for a multi-leveldynamic stabilization assembly utilizing the anchor components of FIGS.1A to 1D according to an embodiment of the present invention.

FIG. 1F is a posterior view of a multi-level dynamic stabilizationassembly utilizing the components of FIGS. 1A to 1E according to anembodiment of the present invention.

FIG. 2A is an exploded view of a deflection rod according to anembodiment of the present invention.

FIG. 2B is a perspective view of the deflection rod assembly of FIG. 2A,as assembled.

FIG. 2C is a sectional view of the deflection rod assembly of FIGS. 2Aand 2B.

FIG. 2D is a sectional view of the deflection rod assembly of FIGS. 2Aand 2B.

FIGS. 2E and 2F are sectional views of the deflection rod assembly ofFIGS. 2A and 2B showing deflection of the post.

FIG. 2G is a transverse sectional view of a vertebra illustrating theimplantation of the deflection rod assembly of FIGS. 2A and 2B.

FIG. 3A is an exploded view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 3B is a perspective view of the deflection rod assembly of FIG. 3A,as assembled.

FIG. 3C is a sectional view of the deflection rod assembly of FIGS. 3Aand 3B.

FIG. 3D is a sectional view of the deflection rod assembly of FIGS. 3Aand 3B showing deflection of the post.

FIG. 3E is a transverse sectional view of a vertebra illustrating theimplantation of the deflection rod assembly of FIGS. 3A and 3B.

FIG. 3F is a transverse sectional view of a vertebra illustrating theimplantation of an alternative deflection rod.

FIG. 3G is a lateral view of a multi-level dynamic stabilizationassembly utilizing the deflection rod assembly of FIGS. 3A-3B accordingto an embodiment of the present invention.

FIG. 3H is an oblique view of an offset connector mounted to thedeflection rod assembly of FIGS. 3A-3B according to an embodiment of thepresent invention.

FIG. 3I shows a socket with interior features adapted to engage featuresof the housing of a deflection rod assembly according to an embodimentof the present invention.

FIG. 3J shows a connector with interior features adapted to engagefeatures of the housing of a deflection rod assembly according to anembodiment of the present invention.

FIG. 4A is an exploded view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 4B is a perspective view of the deflection rod assembly of FIG. 4A,as assembled.

FIG. 4C is a sectional view of the deflection rod assembly of FIGS. 4Aand 4B.

FIG. 4D is a sectional view of the deflection rod assembly of FIGS. 4Aand 4B showing deflection of the post.

FIG. 4E is a sectional view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 4F is a sectional view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 4G is a sectional view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 5A is a graph showing the deflection/force response curves ofvarious embodiments of deflection rods according to embodiments of thepresent invention.

FIG. 5B is a sectional view of an alternative deflection rod.

FIGS. 6A-6H are sectional views of alternative deflection rods accordingto embodiments of the present invention.

FIGS. 7A-7E are perspective views of alternative combinations ofdeflection rods and bone anchors according to embodiments of the presentinvention.

FIGS. 7F-7H are perspective and sectional views of an alternative boneanchors having a torque-limiting breakaway head according to anembodiment of the present invention.

FIGS. 8A-8H show alternative deflection rods having different mechanismsto secure the deflectable post to the deflection rod assembly and/boneanchor.

FIGS. 9A-9C show an alternative deflection rod assembly according to anembodiment of the invention.

FIGS. 9D-9F show an alternative deflection rod assembly according to anembodiment of the invention.

FIGS. 9G and 9H show an alternative deflection rod assembly according toan embodiment of the invention.

FIGS. 10A-10E show a locking hinged mechanism for connecting adeflection rod assembly to a vertical rod according to an embodiment ofthe invention.

FIGS. 11A-11D show a locking ball-joint mechanism for connecting adeflection rod assembly to a vertical rod according to an embodiment ofthe invention.

FIGS. 11E-11F show a locking receiver mechanism attached to a deflectionrod assembly for connecting the deflection rod assembly to a verticalrod according to an embodiment of the invention.

FIGS. 12A-12D show a locking receiver mechanism for connecting avertical rod to a post terminating in a ball according to an embodimentof the invention.

FIGS. 13A and 13B show a deflection rod assembly having a pivoting headaccording to an embodiment of the invention.

FIGS. 14A and 14B show a deflection rod assembly having a pivoting headaccording to an embodiment of the invention.

FIGS. 15A-15C show views of a preferred deflection rod, bone anchor andvertical rod.

FIGS. 16A-16B show perspective views show a deflection rod assemblyhaving an adjustable vertical rod connector according to an embodimentof the invention.

FIG. 17A shows a sectional view of an alternative deflection rodassembly according to an embodiment of the invention.

FIG. 17B shows a perspective view of the sleeve of the deflection rodassembly of FIG. 17A.

FIGS. 17C-17E show views of alternative sleeves for deflection rodassembly according to embodiments of the present invention.

FIG. 18A is an exploded view of an alternative deflection rod assemblyaccording to an embodiment of the present invention.

FIG. 18B is a perspective view of the deflection rod assembly of FIG.18A, as assembled.

FIG. 18C is a sectional view of the deflection rod assembly of FIGS. 18Aand 18B.

FIG. 18D is a sectional view of the deflection rod assembly of FIGS. 18Aand 18B showing deflection of the post.

FIG. 18E is a partial sectional view of the o-ring of FIG. 18A.

FIG. 18F is a partial sectional view of an alternative o-ring.

FIG. 18G is a partial sectional view of another alternative o-ring.

FIG. 18H is a partial sectional view of another alternative o-ring.

FIG. 19A is an exploded view of an alternative deflection rod assemblyaccording to a preferred embodiment of the present invention.

FIG. 19B is a perspective view of the deflection rod assembly of FIG.19A, as assembled.

FIG. 19C is a sectional view of the deflection rod assembly of FIGS. 19Aand 19B.

FIG. 19D is a sectional view of the deflection rod assembly of FIGS. 19Aand 19B showing deflection of the post.

FIGS. 19E-19G show enlarged views of components of the deflection rodassembly of FIGS. 19A-19D.

FIG. 20A shows a perspective view of an implantation tool for a dynamicbone anchor according to an embodiment of the invention.

FIGS. 20B and 20C show detailed sectional views of the head of theimplantation tool of FIG. 20A in relation to a dynamic bone anchor.

FIG. 20D is a transverse view of the lumbar spine illustrating use ofthe implantation tool of FIG. 20A to implant a dynamic bone anchor inthe pedicles of a lumbar vertebra according to an embodiment of theinvention.

FIG. 21A shows a perspective view of an attachment tool for securing adynamic vertical rod to a dynamic bone anchor according to an embodimentof the invention.

FIG. 21B shows a detailed view of the head of the attachment tool ofFIG. 21A.

FIGS. 21C and 21D show detailed sectional views of the head of theattachment tool of FIG. 21A in relation to a dynamic vertical rod andbone anchor.

FIG. 21E-21H are a lateral views of the lumbar spine illustrating stepsto secure a dynamic vertical rod to a dynamic bone anchor assembly usingthe attachment tool of FIG. 21A according to an embodiment of theinvention.

FIG. 22A is a lateral view of the lumbar spine illustrating the naturalkinematics of the spine during extension and flexion.

FIG. 22B is a lateral view of the lumbar spine illustrating thekinematic constraints placed on the spine by a rigid spinal rod systemduring extension and flexion.

FIGS. 22C and 22D show the kinematic modes of an embodiment of thedynamic spine stabilization implant of the invention utilizing a dynamicbone anchor and dynamic vertical rod in accordance with embodiments ofthe invention.

FIG. 22E is a graph illustrating the kinematics of a dynamic spinestabilization prosthesis including the dynamic bone anchor of FIGS. 22Cand 22D.

FIG. 22F is a lateral views of the spine illustrating the kinematics ofthe spine supported by a dynamic spine stabilization prosthesis of FIG.22E.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention includes a versatile spinal implant system andmethods which can dynamically stabilize the spine while providing forthe preservation of spinal motion. Alternative embodiments can be usedfor spinal fusion. An aspect of the invention is restoring and/orpreserving the natural motion of the spine including the quality ofmotion as well as the range of motion. Still, another aspect of theinvention is providing for load sharing and stabilization of the spinewhile preserving motion.

Another aspect of the invention is to provide a modular system which canbe customized to the needs of the patient. Another aspect of embodimentsof the invention is the ability to stabilize two, three and/or morelevels of the spine by the selection of appropriate components forimplantation in a patient. Another aspect of the invention is theability to provide for higher stiffness and fusion at one level or toone portion of the spine while allowing for lower stiffness and dynamicstabilization at another adjacent level or to another portion of thespine. Embodiments of the invention allow for fused levels to be placednext to dynamically stabilized levels. Such embodiments of the inventionenable vertebral levels adjacent to fusion levels to be shielded byproviding a transition from a rigid fusion level to a dynamicallystable, motion preserved, and more mobile level.

Embodiments of the present invention provide for assembly of a dynamicstabilization system which supports the spine while providing for thepreservation of spinal motion. The dynamic stabilization system has ananchor system, a deflection system, a vertical rod system and aconnection system. The anchor system anchors the construct to the spinalanatomy. The deflection system provides dynamic stabilization whilereducing the stress exerted upon the bone anchors and spinal anatomy.The vertical rod system connects different levels of the construct in amultilevel assembly and may in some embodiments include compounddeflection rods. The connection system includes coaxial connectors andoffset connectors which adjustably connect the deflection system,vertical rod system and anchor system allowing for appropriate,efficient and convenient placement of the anchor system relative to thespine. Alternative embodiments can be used for spinal fusion.

Embodiments of the invention include a construct with an anchor system,a deflection system, a vertical rod system and a connection system. Thedeflection system provides dynamic stabilization while reducing thestress exerted upon the bone anchors and spinal anatomy. The anchorsystem anchors the deflection system to the spine. The connection systemconnects the deflection system to the vertical rod system. The verticalrod system connects dynamic stabilization system components on differentvertebra to provide load sharing and dynamic stabilization.

Embodiments of the present invention include a deflection rod assemblywhich provides load sharing while preserving range of motion andreducing stress exerted upon the bone anchors and spinal anatomy. Thedeflection rod assembly includes a deflectable post mounted within abone anchor. Deflection of the deflectable post is controlled by acompliant sleeve. A contact surface of the deflection rod is positionedto limit deflection of the deflectable post. The force/deflectionproperties of the deflection rod assembly may be adapted to the anatomyand functional requirements of the patient.

Common reference numerals are used to indicate like elements throughoutthe drawings and detailed description; therefore, reference numeralsused in a drawing may or may not be referenced in the detaileddescription specific to such drawing if the associated element isdescribed elsewhere. The first digit in a three digit reference numeralindicates the series of figures in which the referenced item firstappears. Likewise the first two digits in a four digit referencenumeral.

The terms “vertical” and “horizontal” are used throughout the detaileddescription to describe general orientation of structures relative tothe spine of a human patient that is standing. This application alsouses the terms proximal and distal in the conventional manner whendescribing the components of the spinal implant system. Thus, proximalrefers to the end or side of a device or component closest to the handoperating the device, whereas distal refers to the end or side of adevice furthest from the hand operating the device. For example, the tipof a bone screw that enters a bone would conventionally be called thedistal end (it is furthest from the surgeon) while the head of the screwwould be termed the proximal end (it is closest to the surgeon).

Dynamic Stabilization System

FIGS. 1A-1F introduce components of a dynamic stabilization systemaccording to an embodiment of the present invention. The componentsinclude anchor system components, deflection rods, vertical rods andconnection system components, including for example coaxial and offsetconnectors. The components may be implanted and assembled to form adynamic stabilization system appropriate for the anatomical andfunctional needs of a patient.

FIG. 1A shows a bone anchor 102 and a deflection rod 104 connected to avertical rod 106 by a ball joint 108. Deflection rod 104 is an exampleof a component of the deflection rod assembly system. Deflection rod 104is a component having controlled flexibility which allows for loadsharing. The deflection rod 104 provides stiffness and support whereneeded to support the loads exerted on the spine during normal spinemotion, which loads, the soft tissues of the spine are no longer able toaccommodate since these spine tissues are either degenerated or damaged.Load sharing is enhanced by the ability to select the appropriatestiffness of the deflection rod in order to match the load sharingcharacteristics desired. For embodiments of this invention, the terms“deflection rod” and “loading rod” can be used interchangeably.Deflection rods, deflection rod mountings and alternative deflectionrods are described in more detail below.

Deflection rod 104 includes a deflectable post 105 which may deflectrelative to a mount 107. Mount 107 is adapted to secure the deflectablepost 105 to bone anchor 102. Mount 107 is received within cavity 132 ofbone anchor 102. When received in cavity 132, mount 107 is secured intoa fixed position relative to bone anchor 102. Deflectable post 105 maystill deflect in a controlled manner relative to bone anchor 102 therebyprovide for load sharing while preserving range of motion of thepatient. The stiffness/flexibility of deflection of the deflectable post105 relative to mount 107/bone anchor 102 may be controlled and/orcustomized as will be described below.

As shown in FIG. 1A, mount 107 is designed to be received within acavity 132 of bone anchor 102. As shown in FIG. 1A, mount 107 includes acollar 140. A threaded aperture 142 extends obliquely through collar140. The threaded aperture 142 receives a locking set screw 144 which,when seated (FIG. 1B), engages the housing 130 of bone anchor 102.Locking set screw 144 is positioned within threaded aperture 142 throughcollar 140. The locking set screw 144 thereby secures the mount 107 ofdeflection rod 104 in place within the housing 130 of bone anchor 102.

As shown in FIG. 1A, deflection rod 104 is oriented in a co-axial,collinear or parallel orientation to bone anchor 102. This arrangementsimplifies implantation, reduces trauma to structures surrounding animplantation site, and reduces system complexity. Arranging thedeflection rod 104, co-axial with the bone anchor 102 can substantiallytransfer a moment (of) force applied by the deflectable post 105 from amoment force tending to pivot or rotate the bone anchor 102 about theaxis of the shaft, to a moment force tending to act perpendicular to theaxis of the shaft. The deflection rod can thereby effectively resistrepositioning of the deflection rod and/or bone anchor 102 without theuse of locking screws or horizontal bars to resist rotation. Furtherexamples of coaxial deflection rods are provided below. Each of thedeflection rods described herein may be used as a component of a dynamicstabilization system.

Bone anchor 102 is an example of a component of the anchor system. Boneanchor 102 includes a bone screw 120 and housing 130. As shown in FIG.1A, bone anchor 102 is a bone screw 120 having one or more threads 124which engage a bone to secure the bone anchor 102 onto a bone. Theanchor system may include one or more alternative bone anchors known inthe art e.g. bone hooks, expanding devices, barbed devices, threadeddevices, adhesive and other devices capable of securing a component tobone instead of or in addition to bone screw 120.

As shown in FIG. 1A, bone anchor 102 includes a housing 130 at theproximal end. Housing 130 includes a cavity 132 for receiving deflectionrod 104. Cavity 132 is coaxial with threaded bone screw 120. Housing 130also comprises a groove 134 for securing deflection rod 104 withinhousing 130. As shown in FIG. 1A, groove 134 is located at the proximalend of housing 130. Groove 134 is designed to be engaged by the lockingmechanism of a component mounted within cavity 132. For example, groove134 is designed to be engaged by locking set screw 144 of deflection rod104. When deflection rod 104 has been positioned within cavity 132 ofbone anchor 102 as shown in FIG. 1B, locking set screw 144 is tightenedto engage groove 134 of housing 130 thus securing deflection rod 104within housing 130. Alternative mechanisms and techniques may be used tosecure the deflection rod to the bone anchor including for example,welding, soldering, bonding, and/or mechanical fittings includingthreads, snap-rings, locking washers, cotter pins, bayonet fittings orother mechanical joints.

Bone anchor 102 also includes a coupling 136 to which other componentsmay be mounted. As shown in FIG. 1A, coupling 136 is the externalcylindrical surface of housing 130. Housing 130 thus provides twomounting positions, one coaxial mounting position and one externalmounting position. Thus, a single bone anchor 102 can serve as themounting point for one, two or more components. A deflection rod 104 maybe coaxially mounted in the cavity 132 of the housing and one or moreadditional components may be externally mounted to the outer surface 136of the housing. For example, a component of the connection system may bemounted to the outer surface 136 of the housing—such a connector may becalled an offset head or offset connector. In some applications acomponent of the connection system may be coaxially-mounted in thecavity 132 in place of a deflection rod 104—such a connector may becalled a coaxial head or coaxial connector.

It is desirable to have a range of different connectors which arecompatible with the anchor system and deflection system. The connectorsmay have different attributes, including for example, different degreesof freedom, range of motion, and amount of offset, which attributes maybe more or less appropriate for a particular relative orientation andposition of two bone anchors and/or patient anatomy. It is desirablethat each connector be sufficiently versatile to connect a vertical rodto a bone anchor in a range of positions and orientations while beingsimple for the surgeon to adjust and secure. It is desirable to providea set of connectors which allows the dynamic stabilization system to beassembled in a manner that adapts a particular dynamic stabilizationassembly to the patient anatomy rather than adapting the patient anatomyfor implantation of the assembly (for example, by removing tissue\boneto accommodate the system). In a preferred embodiment, the set ofconnectors comprising the connection system have sufficient flexibilityto allow the dynamic stabilization system to realize a suitable dynamicstabilization assembly in all situations that will be encountered withinthe defined patient population.

In some embodiments of the present invention, a connection systemcomponent, e.g. a polyaxial connector may be mounted in the cavity 132of a bone anchor 102 to secure the bone anchor to vertical rod 106. Forexample, FIG. 1C shows coaxial head 150 which is a polyaxial connectorwhich is coaxially mounted within the cavity 132 of the housing 130 ofbone anchor 102. Coaxial head 150 is an example of a coaxial head orcoaxial connector. Bone anchor 102 is the same bone anchor previouslydescribed with respect to FIGS. 1A and 1B. Coaxial head 150 comprises arod 152 which is designed to fit within cavity 132 of housing 130.Coaxial head 150 also comprises a collar 154 and locking set screw 156.Locking set screw 156 is configured to engage groove 134 of bone anchor102 in the same way as locking set screw 144 of deflection rod 104. Rod152 and cavity 132 may in some case be circular in section (e.g.cylindrical), in which case rod 152 can rotate within cavity 132 untillocked into place by locking set screw 156. In alternative embodiments,rod 152 may be polygonal in section such that it fits in one of a fixednumber of possible positions.

Referring again to FIG. 1C, attached to rod 152 of coaxial head 150 is ayoke 164. Yoke 164 is connected to a ball 165 by a hexagonal pin 162. Asaddle 163 is also mounted to ball 165 such that saddle 163 can pivotabout two orthogonal axes relative to yoke 164. Saddle 163 has anaperture 168 through which a vertical rod may be passed. On one side ofaperture 168 is a plunger 169. On the other side of aperture 168 is alocking set screw 167. When a vertical rod 106 (not shown) is positionedwithin aperture 168 and locking set screw 167 is tightened down, thelocking set screw 167 forces the vertical rod 106 down onto the plunger169. Plunger 169 is, in turn, forced down by the vertical rod 106against ball 165. Plunger 169 engages ball 165, and ball 165 engageshexagonal pin 162, to lock saddle 163 in position relative to yoke 164and secure a rod (e.g. vertical rod 106) to saddle 163. In this way,tightening set screw 167 secures the vertical rod 106 to the coaxialhead 150 and also locks orientation of the coaxial head 150.

The ability to coaxially mount coaxial head 150 to a bone anchor 102 hasseveral advantages over a standard polyaxial bone screw in which apolyaxial connector is an integral part of the device and may not beremoved or exchanged. The bone anchor 102 is simpler to install andthere is no risk of damage to the polyaxial connector duringinstallation. A single coaxial head 150 can be manufactured and designedto mount to a range of different bone anchors thus allowing bone anchorsto be selected as appropriate for the patient anatomy. After the boneanchor is installed, the orientation of the yoke 164 can be adjustedwithout changing the screw depth (this is not possible in a standardpolyaxial bone screw without also turning the screw). After the boneanchor is implanted, one of a range of different coaxial heads may beinstalled without requiring removal of the bone anchor. Likewise, if arevision is required, the coaxial head may be exchanged for a differentcomponent without necessitating removal of the bone anchor 102.

As described above, bone anchor 102 has housing 130 which can accept onecoaxially-mounted component (e.g. a coaxial head) and oneexternally-mounted component (e.g. an offset connector). FIG. 1D shows acomponent of the connection system which may be mounted externally tohousing 130 of bone anchor 102 in conjunction with a coaxially-mountedcomponent. FIG. 1D shows a perspective view of offset connector 170mounted externally to housing 130 of bone anchor 102 in which adeflection rod 104 is coaxially mounted. Connector 170 may be termed anoffset head or offset connector.

Offset connector 170 comprises six components and allows for two degreesof freedom of orientation and two degrees of freedom of position inconnecting a vertical rod to a bone anchor. The six components of offsetconnector 170 are dowel pin 172, pivot pin 174, locking set screw 176,plunger 178, clamp ring 180 and saddle 182. Saddle 182 has a slot 184sized to receive a rod which may be a vertical rod, e.g. vertical rod106 of FIG. 1A. Locking set screw 176 is mounted at one end of slot 184such that it may be tightened to secure a rod within slot 184.

Clamp ring 180 is sized such that, when relaxed it can slide freely upand down the housing 130 of bone anchor 102 and rotate around housing130. However, when locking set screw 176 is tightened on a rod, theclamp ring 180 grips the housing and prevents the offset connector 170from moving in any direction. Saddle 182 is pivotably connected to clampring 180 by pivot pin 174. Saddle 182 can pivot about pivot pin 174.However, when locking set screw 176 is tightened on a rod, the plunger178 grips the clamp ring 180 and prevents further movement of the saddle182. In this way, operation of the single set screw 176 serves to lockthe clamp ring 180 to the housing 130 of the bone anchor 102, fix saddle182 in a fixed position relative to clamp ring 180 and secure a rodwithin the slot 184 of offset connector 170.

The above-described coaxial connector and offset connector are providedby way of example only. Alternative embodiments of coaxial heads andoffset connectors can be found in U.S. Provisional Patent ApplicationNo. 61/100,625, filed Sep. 26, 2008 entitled “Versatile AssemblyComponents And Methods For A Dynamic Spinal Stabilization System” whichis incorporated by reference. These coaxial heads and offset connectorsmay be used in conjunction with the components herein described topermit assembly of a dynamic stabilization system appropriate to thefunctional needs and anatomy of a particular patient. In addition screwshaving an integrated connector may also be utilized to anchor componentsof the dynamic stabilization system in fixed relationship to a vertebra,for example polyaxial screws.

The components of the dynamic stabilization system may be assembled andimplanted in the spine of a patient to provide a multilevel dynamicstabilization assembly which provides dynamic stabilization of the spineand load sharing. In some embodiments, the first step is implantation ofbone anchors in the vertebrae. In other embodiments, the bone anchorsmay be implanted with the deflection rod/connection component alreadyinstalled.

FIG. 1E, shows three adjacent vertebrae 191, 192 and 193. As apreliminary step, bone anchors 102 a, 102 b and 102 c have beenimplanted in the vertebrae 191, 192 and 193 on the right side of thespinous process 194 between the spinous process 194 and the transverseprocess 195. A driver is inserted into the cavity 132 a, 132 b, 132 c inorder to drive the threaded portion of each bone anchor into the bone.In preferred procedures, the bone anchor is directed so that thethreaded portion is implanted within one of the pedicles 196 angledtowards the vertebral body 197. The threaded region of each bone anchoris fully implanted in the vertebrae 191, 192 and 193. A driver mayalternatively and/or additionally engage the exterior surface of housing130 in order to implant the bone anchor. The driver may have atorque-measuring and/or torque limiting function to assist in accurateimplantation of the bone screw and avoid excess force being applied tothe vertebrae. In alternative embodiments, the bone screw mayincorporate a torque limiting element, for example a secondary headwhich breaks away when the driver torque exceeds a predetermined torquelimit. See, e.g. FIGS. 7F-7H and accompanying text.

As shown in FIG. 1E, the housings 130 a, 130 b, 130 c of each boneanchor remain partly or completely exposed above the surface of thevertebrae so that one or more of a connection system component anddeflection component can be secured to each bone anchor 102 a, 102 b and102 c. Coaxial components may be coaxially-mounted inside each ofcavities 132 a, 132 b, and 132 c. Offset heads/connectors may also beexternally-mounted to the outside surface of each of housings 130 a, 130b and 130 c. Note that bone anchors are also implanted on the left sideof the spine.

After installation of the bone anchors, the deflection systemcomponents, vertical rod systems components and connection systemcomponents may be installed and assembled. FIG. 1F shows one way toassemble deflection system components and connection system components.As shown in FIG. 1F, a coaxial head 150 is installed in bone anchor 102c. An offset connector 170 is mounted externally to the housing of boneanchor 102 b. A deflection rod 104 a is coaxially mounted in the housingof bone anchor 102 a. A deflection rod 104 b is coaxially mounted in thehousing of bone anchor 102 b. A vertical rod 106 a is connected at oneend to deflection rod 104 a by ball joint 108 a. Vertical rod 106 a isconnected at the other end by in-line connector 170 to bone anchor 102b. A second vertical rod 106 b is connected at one end to deflection rod104 b by ball joint 108 b. Vertical rod 106 b is connected at the otherend by coaxial head 150 to bone anchor 102 c.

The dynamic stabilization assembly 190 of FIG. 1E thus has a verticalrod 106 a, 106 b stabilizing each spinal level (191-192 and 192-193).Each of the vertical rods 106 a, 106 b is secured rigidly at one end toa bone anchor (102 b, 102 c). Each of the vertical rods 106 a, 106 b issecured at the other end by a ball joint 108 a, 108 b to a deflectionrod 104 a, 104 b thereby allowing for some movement and load sharing bythe dynamic stabilization assembly. Offset connector 170 and coaxialhead 150 permit assembly of dynamic stabilization assembly 190 for awide range of different patient anatomies and/or placements of boneanchors 102 a, 102 b and 102 c. An identical or similar dynamicstabilization assembly would preferably be implanted on the left side ofthe spine. It should be noted that dynamic stabilization assembly 190does not require horizontal bars or locking screws thereby reducing theexposure of tissue and/or bone to foreign bodies compared to systemswith this additional hardware. The dynamic stabilization assembly ofFIG. 1F, thereby, has a small footprint, potentially reducing the amountof displacement of tissue and/or bone, reducing trauma to tissue and/orbone during surgery. Further, the smaller footprint can reduce theamount of tissue that needs to be exposed during implantation.

The particular dynamic stabilization assembly shown in FIG. 1G isprovided by way of example only. It is an aspect of preferredembodiments of the present invention that a range of components beprovided and that the components may be assembled in differentcombinations and organizations to create different assemblies suitablefor the functional needs and anatomy of different patients. Also,deflection rods having different force deflection characteristics may beincorporated at different spinal levels in accordance with theanatomical and functional requirements. Dynamic stabilization may beprovided at one or more motion segments and in some cases dynamicstabilization may be provided at one or more motion segments inconjunction with fusion at an adjacent motion segment. Particulardynamic stabilization assemblies may incorporate combinations of thebone anchors, vertical rods, deflection rods, offset and coaxialconnectors described herein, in the related applications incorporated byreference, and standard spinal stabilization and/or fusion components,for example screws, rods and polyaxial screws.

Deflection Rods/Loading Rods

One feature of embodiments of the present invention is the load sharingand range of motion provided by a deflection rod. The deflection rodprovides stiffness and support where needed to support the loads exertedon the spine during normal spine motion thereby recovering improvedspine function without sacrificing all motion. The deflection rod alsoisolates the anchor systems components from forces exerted by thedynamic stabilization assembly thereby reducing stress on the boneanchors and the bone to which they are attached. Moreover, by selectingthe appropriate stiffness of the deflection rod or loading rod to matchthe physiology of the patient and the loads that the patient places onthe spine, a better outcome is realized for the patient.

The deflection rod includes a deflectable post, a compliant sleeve and amount. The deflectable post and mount are typically made ofbiocompatible metal or metals, e.g. titanium and stainless steel. Thesleeve is made of a compliant material, for example a compliant polymer.The mount secures the deflection rod to an anchoring device in a mannerwhich allows deflection of the deflectable post. The deflectable post isconfigured to connect to the vertical rod system. The deflectable postmay deflect relative to the mount by compressing the compliant materialof the sleeve. The deformation of the sleeve imparts force/deflectioncharacteristics to the deflectable post. The movement of the postrelative to the mount allows controlled movement of the bone anchor (andvertebra in which it is implanted) relative to the vertical rods therebysupporting the vertebrae to which the bone anchors are attached whileallowing movement of the vertebrae.

Deflection rods can be manufactured in a range from highly rigidconfigurations to very flexible configurations by appropriate selectionof the design, materials and dimensions of the post, sleeve and mount.Deflection rods having a particular stiffness/flexibility may beselected for use in a dynamic stabilization assembly based upon thephysiological needs of a particular patient. In a preferred embodimentdeflection rod stiffness/flexibility is selected to provide load sharingin conjunction with from 50% to 100% of the normal range of motion of apatient and more preferably 70% to 100% of the normal range of motion ofa patient.

In some cases, certain of the deflection rods of a dynamic stabilizationassembly can have a different stiffness or rigidity or flexibility thanother of the deflection rods. Thus, in the same assembly, a firstdeflection rod can have a first flexibility or stiffness or rigidity,and a second deflection rod can have a second different flexibility orstiffness or rigidity depending on the needs of the patient. Particularembodiments of a dynamic stabilization assembly may utilize deflectionrods having different deflection properties for each level and/or sideof the dynamic stabilization assembly. In other words, one portion of adynamic stabilization assembly may offer more resistance to movementthan the other portion based on the design and selection of differentstiffness characteristics, if that configuration benefits the patient.

FIGS. 2A through 2G illustrate the design and operation of a firstembodiment of a deflection rod according to an embodiment of the presentinvention. FIG. 2A shows an exploded view of deflection rod 200.Deflection rod 200 includes retainer 202, deflectable post 204, sleeve206, shield 208, collar 210, screw 212 and ball 214. Deflection rod 200connects to vertical rod 216 at a ball joint which includes ball 214,pocket 218 and cap 220. Shield 208 and collar 210 are securely attachedto each other (or formed in one piece) and make up the mount 207. Athreaded aperture 211 passes obliquely through collar 210. Threadedaperture 211 is configured to receive a screw 212. Sleeve 206 is made ofa compliant material which permits movement of deflectable post 204relative to shield 208. Deflectable post 204 may thus pivot in anydirection about the center of ball-shaped retainer 202 as shown byarrows 230. The sleeve 206 controls and limits the deflection of thedeflectable post 204. The deflectable post 204 can also rotate about thelongitudinal axis of the post and the bone anchor as shown by arrow 232.

Referring now to FIG. 2B, which shows a perspective view of a fullyassembled deflection rod 200. When assembled, deflectable post 204 ispositioned within sleeve 206; sleeve 206 is positioned within shield208. Ball 214 is connected to the proximal end of deflectable post 204to provide a component of a ball joint for connecting deflection rod 200to a vertical rod 216. Ball 214 may be formed in one piece withdeflectable post 204 or may be securely attached to deflectable post 204using a joint, for example, a threaded joint, welded joint, adhesivejoint. Retainer 202 is attached to the distal end of deflectable post204 to prevent deflectable post 204 from being pulled out of sleeve 206.

As shown in FIG. 2A, the retainer 202 may be a ball-shaped retainer 202.Retainer 202 may be formed in one piece with deflectable post 204 or maybe securely attached to deflectable post 204. The retainer 202 may beattached by laser welding, soldering or other bonding technology. Forexample, retainer 202 in the form of a ball, disk, plate or other shapemay be laser welded to the distal end of deflectable post 204.Alternatively, retainer 202 may mechanically engage the deflectable post204 using, for example, threads. For example, a lock ring, toothedlocking washer, cotter pin or other mechanical device can be used tosecure deflectable post 204 within shield 208.

The ball 214 of deflection rod 200 is received in a pocket of verticalrod 216. Cap 220 secures ball 214 within the pocket of vertical rod 216creating a ball joint 222 which allows vertical rod 216 to rotate 360degrees around the axis of deflectable post 204 (as shown by arrow 234)and also tilt away from the plane perpendicular to the axis ofdeflectable post 204 (as shown by arrow 236). Thus, the vertical rod 216is allowed to rotate and/or have tilting and/or swiveling movementsabout a center which corresponds with the center of ball 214 of balljoint 222. Ball 214 can also be displaced relative to shield 208 bydeflection of deflectable post 204 (as shown by arrows 230).

FIG. 2C shows a sectional view of a fully assembled deflection rod 200along the axis indicated by line C-C of FIG. 2B. As shown in FIG. 2C,sleeve 206 occupies the space between deflectable post 204 and shield208 and is compressed by deflection of deflectable post 204 towardsshield 208 in any direction. In some embodiments, sleeve 206 may beformed separately from deflection rod 200. For example, deflectable post204 and sleeve 206 may be press fit into shield 208. Alternatively oradditionally, a biocompatible adhesive may be used to bond the sleeve206 to the shield 208 and/or deflectable post 204. Alternatively, sleeve206 may be formed in place by positioning the deflectable post 204within the shield 208 and then filling the space between the deflectablepost 204 and the shield 208 with liquid polymer (polymer reagents) andallowing the polymer to solidify (polymerize).

FIG. 2C, also illustrates the internal detail of ball joint 222 whichconnects vertical rod 216 and deflectable post 204 of deflection rod200. Vertical rod 216 includes disk-shaped pocket 218 at one end. Theproximal end of deflectable post 204 is passed through aperture 219 indisk-shaped pocket 218 of the vertical rod 216. The diameter ofdeflectable post 204 is smaller than the diameter of aperture 219. Oncethe proximal end of deflectable post 204 is passed through the aperture219, ball 214 is attached to deflectable post 204 using threading,fusing, gluing, press fit and/or laser welding techniques, for example.The diameter of the aperture 219 is less than the diameter of ball 214to prevent ball 214 from passing back through aperture 219. Once ball214 is positioned within the disk-shaped pocket 218 of vertical rod 216,cap 220 is threaded, fused, glued, press fit and/or laser welded, forexample, into pocket 218 thereby securing ball 214 within disk shapedpocket 218. FIG. 2C also shows an optional ridge 209 on the interior ofshield 208 for retaining sleeve 206.

FIG. 2D shows a sectional view of a fully assembled deflection rod 200along the axis indicated by line D-D of FIG. 2B. As shown in FIG. 2D,sleeve 206 occupies the space between deflectable post 204 and shield208 and is compressed by deflection of deflectable post 204 towardsshield 208 in any direction. Sleeve 206 resists deflection ofdeflectable post 204 outwardly from a position that is collinear withthe longitudinal axis of sleeve 206. The dimensions and material ofsleeve 206 may be adjusted to generate the desired deflection/loadcharacteristics for the deflection rod.

FIGS. 2E and 2F illustrate deflection of deflectable post 204. Applyinga force to ball-joint 222 causes deflection of deflectable post 204relative to mount 207 including shield 208 (and any bone anchor to whichit may be mounted). Initially deflectable post 204 pivots about a pivotpoint 203 indicated by an X. In this embodiment pivot point 203 islocated at the center of ball-shaped retainer 202. In other embodimentshowever, pivot point may positioned at a different location. As shown inFIG. 2E, deflection of deflectable post 204 initially compresses thematerial of sleeve 206 between deflectable post 204 and shield 208. Theforce required to deflect deflectable post 204 depends upon thedimensions of deflectable post 204, sleeve 206 and shield 208 as well asthe attributes of the material of sleeve 206.

By changing the dimensions of deflectable post 204, sleeve 206 andshield 208, the deflection characteristics of deflection rod 200 can bechanged. The stiffness of components of the deflection rod can be, forexample, increased by increasing the diameter of the post and/or bydecreasing the diameter of the inner surface of the shield anddeflection guide. Additionally, increasing the diameter of the post willincrease the stiffness of the deflection rod while decreasing thediameter of the post will decrease the stiffness of the deflection rod.Alternatively and/or additionally changing the materials which comprisethe components of the deflection rod can also affect the stiffness andrange of motion of the deflection rod. For example, making sleeve 206out of stiffer and/or harder material reduces deflection of deflectablepost 204.

The stiffness of the deflection rod may thus be varied or customizedaccording to the needs of a patient. The deflection characteristics ofthe deflection rod can be configured to approach the natural dynamicmotion of the spine, while giving dynamic support to the spine in thatregion. It is contemplated, for example, that the deflection rod can bemade in stiffness that can replicate a 70% range of motion andflexibility of the natural intact spine, a 50% range of motion andflexibility of the natural intact spine and a 30% range of motion andflexibility of the natural intact spine. In some cases, a kit isprovided to a doctor having a set of deflection rods with differentforce/deflection characteristics from which the doctor may select thedeflection rods most suitable for a particular patient. In other cases,the surgeon may select deflection rods prior to the procedure based uponpre-operative assessment.

Sleeve 206 is preferably made of a compliant biocompatible polymer.Sleeve 206 may, for example, be made from a polycarbonate urethane (PCU)such as Bionate®. If the sleeve is comprised of Bionate®, apolycarbonate urethane or other hydrophilic polymer, the sleeve can alsoact as a fluid-lubricated bearing for rotation of the deflectable post204 relative to the longitudinal axis of the deflectable post 204 (seearrow 232 of FIG. 2B). In a preferred embodiment, the sleeve is made ofPCU, is 2 mm thick when uncompressed and may be compressed to about 1 mmin thickness by deflection of the post.

The sleeve may also include polymer regions having different properties.For example, the sleeve can include concentric rings of one or morepolymers with each ring having a different hardness of stiffness ordurometer. For example, each successive ring from the center outward canhave a higher hardness or stiffness or durometer so that as the post isdeflected outwardly from a position that is collinear with thelongitudinal axis of the sleeve provides increased resistance to furtherdeflection. The sleeve may also be designed to provide different forcedeflection characteristics in different directions. The deflectable postcould also be designed so that less resistance occurs with increaseddeflection of the post.

As shown in FIG. 2F, after further deflection, deflectable post 204comes into contact with limit surface 228 of shield 208. Limit surface228 is oriented such that when deflectable post 204 makes contact withlimit surface 228, the contact is distributed over an area to reducestress on deflectable post 204 and limit surface 228. As depicted, limitsurface 228 is configured such that as the deflectable post 204 deflectsinto contact with limit surface 228, limit surface 228 is aligned/flatrelative to deflectable post 204 in order to present a larger surface toabsorb any load and also to reduce stress on deflectable post 204 andlimit surface damage. Additional deflection may cause elasticdeformation of deflectable post 204. Because deflectable post 204 isrelatively stiff, the force required to deflect deflectable post 204increases significantly after contact of deflectable post 204 withshield 208. In a preferred embodiment, deflectable post 204 may deflectfrom 0.5 mm to 2 mm in any direction before making contact with limitsurface 228. More preferably, deflectable post 204 may deflectapproximately 1 mm before making contact with limit surface 228.

Thus, as load or force is first applied to the deflection rod by thespine, the deflection of the deflection rod responds about linearly tothe increase in the load during the phase when deflection of deflectablepost 204 causes compression of sleeve 206 as shown in FIG. 2E. Afterabout 1 mm of deflection, when deflectable post 204 contacts limitsurface 228 (as shown in FIG. 2F) the deflection rod becomes stiffer.Thereafter, a greater amount of load or force needs to be placed on thedeflection rod in order to obtain the same incremental amount ofdeflection that was realized prior to this point because furtherdeflection requires bending of deflectable post 204. Accordingly, thedeflection rod provides a range of motion where the load supportedincreases about linearly as the deflection increases and then withincreased deflection the load supported increases more rapidly in anon-linear manner in order to provide stabilization. Put another way,the deflection rod becomes stiffer as the deflection/load increases. Ina dynamic stabilization assembly incorporating the deflection rod, theload sharing and deflection is provided by the deflection rod betweenthe deflectable post and the bone screw or the overall bone anchor suchas bone anchor 102 and to a lesser degree or not in the vertical rodsuch as the vertical rod 106 (FIG. 1B).

FIG. 2G is a sectional view illustrating the implantation of deflectionrod 200 in a vertebra 240. As shown in FIG. 2G, bone anchor 102 isoriented such that is passes through pedicle 242 into vertebral body244. Note that the length of bone anchor 102 is selected based upon theanatomy of the patient. Thus shorter bone anchors are used in smallervertebrae and longer bone anchors are used in larger vertebrae. As shownin FIG. 2G, bone anchor 102 has shallower threads 250 adjacent housing130. Threads 250 engage the harder cortical bone 246 on the surface ofthe vertebra 240. Bone anchor 102 has deeper threads 252 towards thedistal end of bone anchor 102. Threads 252 engage the softer cancellousbone 248 within the vertebral body 244.

As shown in FIG. 2G, deflection rod 200 is mounted within bone anchor102 such that pivot point 203 is positioned below the surface ofvertebra 240. Deflectable post 204 pivots about this pivot point 203positioned within vertebra 240. This is advantageous in that it placespivot point 203 of deflectable post 204 closer to the vertebral body 244and thus closer to the natural instantaneous center of rotation of thespine. Placing pivot point 203 closer to the vertebral body 244 promotesnatural motion and reduces non physiological forces on the bones andstrain on the system. Placing the pivot point 203 closer to thevertebral body 244 also helps isolate bone anchor 102 from the relativemotion between vertebra 240 and the vertical rod 216 which connects onevertebra to another vertebra. Pivot point 203 is preferably at or belowthe surface of the vertebra and more preferably pivot point 203 iswithin the cancellous bone 248 of the vertebrae 240. Even morepreferably, the pivot point 203 is positioned with the pedicle 242 ofthe vertebra 240. In some cases, pivot point 203 may be positionedwithin vertebral body 244.

Alternative Deflection Rods/Loading Rods

FIGS. 3A-3H illustrate a first alternative deflection rod 300. FIG. 3Ashows an exploded view of alternative deflection rod 300. Deflection rod300 includes ball-shaped retainer 302, deflectable post 304, sleeve 306,shield 308, collar 310, and mount 314. In this embodiment, retainer 302is a spherical structure formed in one piece with deflectable post 304.Mount 314, in this embodiment, is the proximal end of deflectable post304 suitable for connecting to a vertical rod. A ball may be used inplace of mount 314 as previously described. In this embodiment, mount314 is formed in one piece with deflectable post 304 and retainer 302.In alternative embodiments, deflectable post 304 may be formedseparately from and securely attached to one or more of mount 314 andretainer 302 by laser welding, soldering or other bonding technology.Alternatively, deflectable post 304 may be formed separately andmechanically engage one or more of mount 314 and retainer 302 using, forexample, threads. For example, a lock ring, toothed locking washer,cotter pin or other mechanical device can be used to secure deflectablepost 304 to one or more of mount 314 and retainer 302.

Sleeve 306 is made of a compliant material which permits movement ofdeflectable post 304 relative to shield 308. The sleeve 306 effectivelycontrols and limits the deflection of the deflectable post 304. Sleeve306 is preferably made of a compliant biocompatible polymer such as PCUby way of example only. The properties of the material and dimensions ofthe sleeve 306 are selected to achieve the desired force/deflectioncharacteristics for deflectable post 304. In a preferred embodiment, thesleeve is made of PCU (Bionate® 80A) and is 2 mm thick when uncompressedand may be compressed to about 1 mm in thickness by deflection of thepost. Sleeve 306 may also be shaped to modify the compliance of sleeve306, for example by providing flutes 307. Sleeve 306 fits inside shield308 surrounding deflectable post 304.

Deflection rod 300 is configured to be mounted in a bone anchor 320,which comprises a bone screw 322 connected to a housing 330. Housing 330has a cavity 332 oriented along the axis of bone anchor 320 at theproximal end and configured to receive deflection rod 300. Housing 330also has an outer surface 334 adapted for mounting a component e.g. anoffset connector. Housing 330 may in some embodiments be cylindrical aspreviously described. As shown in FIG. 3A, outer surface 334 of housing330 is provided with splines/flutes 336. Splines/flutes 336 may beengaged by a driver that mates with splines/flutes 336 for implantingbone anchor 320.

Referring now to FIG. 3B, which shows a perspective view of a deflectionrod 300 assembled with a bone anchor 320. When assembled, deflectablepost 304 is positioned within sleeve 306 of FIG. 3A; sleeve 306 ispositioned within shield 308 of FIG. 3A. Deflectable post 304, sleeve306 and shield 308 are then placed in the cavity 332 of FIG. 3A of boneanchor 320. Threaded collar 310 is then secured in the threaded proximalend of cavity 332. Threaded collar 310 has two sockets 311 for receivingthe pins of a pin wrench to allow threaded collar 310 to be tightened tothreads 338 of housing 330. Threaded collar 310 is laser welded tohousing 330 after installation to further secure the components.Threaded collar 310 secures deflectable post 304, sleeve 306 and shield308 within cavity 332 of bone anchor 320.

FIG. 3C shows a sectional view of a deflection rod 300 assembled with abone anchor 320 along the axis indicated by line C-C of FIG. 3B. Asshown in FIG. 3C, sleeve 306 occupies the space between deflectable post304 and shield 308 and is compressed by deflection of deflectable post304 towards shield 308 in any direction. Retainer 302 fits into ahemispherical pocket 339 in the bottom of cavity 332 of housing 330.Shield 308 includes a flange 309 which secures ball-shaped retainer 302within hemispherical pocket 339 while allowing rotation of ball-shapedretainer 302. Collar 310 secures both shield 308 and sleeve 306 withinhousing 330. If sleeve 306 is comprised of Bionate®, a polycarbonateurethane or other hydrophilic polymer, sleeve 306 can act as a fluidlubricated bearing and allow the post to also rotate about thelongitudinal axis of the post and the bone anchor. Other materials andconfigurations can also allow the post to rotate about the longitudinalaxis of the post and the bone anchor.

FIG. 3D illustrates the deflection of deflectable post 304. Applying aforce to mount 314 causes deflection of deflectable post 304 ofdeflection rod 300. Initially deflectable post 304 pivots about a pivotpoint 303 indicated by an X. Deflectable post 304 may pivot about pivotpoint 303 in any direction. Concurrently or alternatively, deflectablepost 304 can rotate about the long axis of deflectable post 304 (whichalso passes through pivot point 303). In this embodiment, pivot point303 is located at the center of ball-shaped retainer 302. As shown inFIG. 3D, deflection of deflectable post 304 initially compresses thematerial of sleeve 306. The force required to deflect deflectable post304 depends upon the dimensions of deflectable post 304, sleeve 306 andshield 308 as well as the attributes of the material of sleeve 306.

After further deflection, deflectable post 304 comes into contact withlimit surface 313 of collar 310. Limit surface 313 is oriented such thatwhen deflectable post 304 makes contact with limit surface 313, thecontact is distributed over an area to reduce stress on deflectable post304. After deflectable post 304 comes into contact with limit surface313, further deflection requires deformation (bending) of deflectablepost 304. In a preferred embodiment, deflectable post 304 is a titaniumpost 5 mm in diameter. Deflectable post 304 is relatively stiff, and theforce required to deflect deflectable post 304 therefore increasessignificantly after contact of deflectable post 304 with collar 310. Ina preferred embodiment, deflectable post 304 may deflect from 0.5 mm to2 mm in any direction before making contact with limit surface 313. Morepreferably, deflectable post 304 may deflect approximately 1 mm beforemaking contact with limit surface 313.

The inner diameter of the collar 310 may be different in differentcollars so that the distance between limit surface 313 and deflectablepost 304 is different in different deflection rods. This allows for themanufacture of deflection rods having a larger or smaller range ofdeflection before contact between the post and the limit surface. Inthis way, deflection rods may be manufactured having different ranges ofmotion. Moreover, the distance between limit surface 313 and deflectablepost 304 need not be the same in all directions such that the range ofmotion of the deflection rod is different in different directions.

Referring to FIG. 3D, as load or force is first applied to thedeflection rod 300 by the spine, the deflection of deflectable post 304responds about linearly to the increase in the load during the phasewhen deflection of deflectable post 304 causes compression of sleeve306. After about 1 mm of deflection, deflectable post 304 contacts limitsurface 313 and the deflection rod becomes substantially stiffer. Agreater amount of load or force needs to be placed on the deflection rodin order to obtain the same amount of incremental deflection that wasrealized prior to this point because further deflection requires bendingof deflectable post 304. The amount of deflection caused by the loadapplied is a non-linear function, in this embodiment. The deflection rodprovides a range of motion where the load supported increases aboutlinearly as the deflection increases and then with increased deflectionthe load supported increases more rapidly (upon contact of the post withthe limit surface). Alternatively, if desired, this embodiment could bedesigned such that the rate of change of the amount of deflection couldbe a linear function for a larger range of motion by; for example,increasing the distance between limit surface 313 and deflectable post304.

FIG. 3E is a sectional view illustrating the implantation of adeflection rod 300 in a vertebra 240. As shown in FIG. 3E, bone anchor320 is oriented such that is passes through pedicle 242 into vertebralbody 244. Note that the length of bone anchor 320 is selected based uponthe anatomy of the patient. Thus shorter bone anchors are used insmaller vertebrae and longer bone anchors are used in larger vertebrae.As shown in FIG. 3E, housing 330 of bone anchor 320 is mounted entirelyabove the surface of vertebra 240. Pivot point 303 of deflection rod 300is positioned within housing 330 such that pivot point 303 is, in thisembodiment, positioned close to but outside of vertebra 240.

In an alternative embodiment, as shown in FIG. 3F, deflectable post 304pivots about a pivot point 353 positioned within vertebra 240. This isadvantageous in that it places pivot point 353 of deflectable post 304closer to the vertebral body 244 and thus the natural instantaneouscenter of rotation of the spine. Placing the pivot point 353 closer tothe vertebral body 244 promotes natural motion and reducesnon-physiological forces on the bones and strain on the dynamicstabilization assembly. In particular, placing the pivot point 353closer to the vertebral body 244 helps isolate bone anchor 320 from therelative motion between vertebra 240 and a vertical rod of the dynamicstabilization assembly which connects one level of the spine to theadjacent level. Pivot point 353 is preferably at or below the surface ofthe vertebra 240. More preferably, the pivot point 353 is positionedwith the pedicle 242 of the vertebra 240. In some cases, pivot point 353may be positioned within vertebral body 244.

FIG. 3G shows a lateral view of a dynamic stabilization assemblyutilizing deflection rod 300. As shown in FIG. 3G, deflection rod 300 isinstalled in bone anchor 320. Bone anchor 320 is implanted in onevertebra 370 (see e.g. FIG. 3E). A polyaxial screw 350 is implanted in asecond vertebra 372. A vertical rod 360 is secured at one end to mount314 of deflection rod 300. Mount 314 in this embodiment passes throughan aperture in vertical rod 360. The proximal end of mount 314 isthreaded so that vertical rod 360 may be secured to mount 314 with athreaded nut 362. In this embodiment, as shown in FIG. 3G, the verticalrod 360 is secured rigidly to deflectable post 304. The rigid connectionprovides a relatively stiff assembly. However, where greater range ofmotion is desired, deflectable post 304 may be provided with a ball endand vertical rod 360 may be connected to deflectable post 304 by a balljoint as previously described with respect to FIGS. 1A-1B.

Vertical rod 360 is mounted at the other end to the polyaxial head 352of polyaxial screw 350. This screw may be a standard polyaxial screw,for example, a 5.5 mm polyaxial screw available in the marketplace. Thisscrew may, alternatively, be a bone anchor with a polyaxial head e.g.the polyaxial head previously described with respect to FIG. 1C. In apreferred embodiment, vertical rod 360 is a titanium rod 5.5 mm indiameter as used in rigid spinal implants. The vertical rod 360 issecured to polyaxial head 352 using a threaded fitting, set screw 354,for example. The vertical rod 360 thereby supports the vertebrae whiledeflection rod 300 provides for load sharing and allows relative motionof vertebra 370 relative to vertebra 372. Thus, the dynamicstabilization assembly provides dynamic stabilization of the spine. Thedynamic stabilization assembly may be expanded to two or more levelsusing an offset connector mounted to the housing 330 of bone anchor 320.It is to be understood that an offset connector can include a flutedring to assist in engaging the housing 330 (see e.g. shape of openwrench 380 in FIG. 3H). Thus, a modular system is provided whichprovides for the creation of a multi-level dynamic stabilizationassembly.

FIG. 3H illustrates an open wrench 380 for driving bone anchor 320 intoposition. Bone anchor 320 of FIG. 3H has a housing 330. A deflection rod300 is installed in housing 330 and secured in place by threaded collar310 (FIGS. 3A and 3B). Threaded collar 310 engages threads interior tohousing 330. Collar 310 has two apertures 311 which may be engaged by apin wrench to tighten collar 310 to housing 330. Collar 310 may also bewelded to housing 330 to further secure deflection rod 300 with housing330. In this embodiment deflection rod 300 is designed to bepreassembled with bone anchor 320 prior to implantation.

As shown in FIG. 3H, the exterior surface 334 of housing 330 is providedwith surface features in the form of a plurality of splines 336. Splines336 are oriented parallel to the longitudinal axis of bone anchor 320and project from housing 330 at regular intervals. Open wrench 380 has ahead 382 designed to engage the exterior surface 334 of housing 330.With such a tool, the housing 330 can be engaged and rotated about thelongitudinal axis of the bone anchor 320 in order to drive the boneanchor into the bone. Open wrench 380 may be provided with a torquelimiting or torque measuring component to facilitate installation ofbone anchor 320. In alternative embodiments a socket may be used toengage housing 330 in place of an open wrench.

FIG. 3I shows a plan view of bone anchor 320 and deflection rod 300observed from the deflection rod end of the assembly. As shown in FIG.3I there are 16 splines 336 evenly spaced around the exterior surface334 of housing 330. The diameter of collar 310 is the same or smaller asthe minimum diameter of housing 330 in the region of the splines 336 toallow engagement of the splines 336 by a complementary tool or connectorwithout interference from collar 310. In other embodiments there may bea greater or lesser number of splines.

FIG. 3I shows a sectional view of a socket wrench 384 suitable forengaging housing 330. Socket wrench 384 has a plurality of splines 386complementary to splines 336 of housing 330. Socket wrench 384 maytherefore be slipped over deflection rod 300 and housing 330 andpositioned as shown in FIG. 3I. When in position, socket wrench 384 maybe used to rotate housing 330 to install bone anchor 320 in a bone (orremove the bone anchor from the bone). Socket wrench 384 should becomplementary in interior profile to the exterior profile 334 of housing330. Socket wrench 384 need not have as many splines 386 as housing 330has splines 336 so long as splines 386 are correctly positioned toengage some or all of the splines 336 of housing 330. An open wrench orother driver may be designed with the same engagement surface to engagesome or all of the splines 336 of housing 330.

Likewise, connectors that engage the housing of a bone anchor may alsobe readily adapted to engage splines 336 of housing 330. By way ofexample, FIG. 3J shows connector 170 of FIG. 1D adapted to engagesplines 336. Connector 170 mounts externally of the housing 330 of abone anchor 320. The components of connector 170 shown in FIG. 3Jinclude locking set screw 176, clamp ring 180 and saddle 182. As shownin FIG. 3J, clamp ring 180 has, on the inside diameter, a plurality ofsplines 396 complementary to splines 336 of housing 330. Clamp ring 180may therefore be slipped over deflection rod 300 and housing 330 andpositioned as shown in FIG. 3J after implantation of bone anchor 320 ina vertebra. Splines 396 engage splines 336 of housing 330. Clamp ring180 is prevented by splines 396 and 336 from free rotation aroundhousing 330. This is advantageous in that increases the stability of thedynamic stabilization assembly by preventing the clamp ring 180 fromslipping around housing 330 under load. When clamp ring 180 ispositioned at the desired angle relative to bone anchor 320, set screw176 may be tightened onto a vertical rod (not shown) to clamp thevertical rod to the saddle 182 and also tighten clamp ring 180 againstthe exterior surface 334 of housing 330. Thus connector 180 may be usedto securely attach a vertical rod to the housing 330 of bone anchor 320.

Clamp ring 180 (and thus connector 170) may be installed in any of 16positions around housing 330 (22.5 degrees separation betweenpositions). If smaller granularity of positioning is required, a largernumber of splines 336 may be used. Clamp ring 180 should becomplementary in interior profile to the exterior surface 334 of housing330. Clamp ring 180 need not have as many splines 396 as housing 330 hassplines 336 so long as the splines 396 are correctly positioned toengage some or all of the splines 336 of housing 330. A clamp ring 180as shown in FIG. 1D without any splines may still be used to engagehousing 330.

Other connectors may be similarly adapted to engage the splines 336 ofhousing 330 of bone anchor 320. Likewise, the other bone anchorsdiscussed herein may be provided with splines on the exterior of thehousing to facilitate installation and enhance the mounting ofconnectors. In alternative embodiments, different surface features maybe utilized on the surface of a housing for engagement by a tool orconnector. For example, a housing may be made polygonal in exteriorsection and have 8, 3, 12, 16 or more sides. A tool or connector for usewith such a housing would have a complementary interior profile designedto engage the 8, 3, 12, 16 or more sides. Alternatively, a housing maybe provided with a plurality of apertures at regular intervals. A toolor connector for use with such a housing may be provided with a one ormore of pins designed to engage the apertures in a plurality ofpositions in the manner of a pin wrench. Conversely the housing may beprovided with one or more protruding pins and the tool or connector witha plurality of complementary apertures. Alternatively, one or both ofthe housing and connector may be provided with shallow surface featuressuch as dots, dimples, ridges or the like designed to increase thefrictional engagement of the housing and connector. In the latter case,the features of the housing and connector need not necessarily becomplementary to one another and the connector and housing may be freeto engage one another at any angular position.

FIGS. 4A-4D illustrate a second alternative deflection rod 400. FIG. 4Ashows an exploded view of an alternative deflection rod 400. Deflectionrod 400 includes retainer 402, deflectable post 404, sleeve 406, shield408, collar 410, and mount 414. In this embodiment, retainer 402 is aball-shaped structure formed in one piece with deflectable post 404.Mount 414 is suitable for connecting to a vertical rod. A ball may beused in place of mount 414 as previously described. In this embodiment,mount 414 is formed in one piece with deflectable post 404. In apreferred embodiment, mount 414, ball-shaped retainer 402 anddeflectable post 404 are formed from a single piece of titanium. Inalternative embodiments, deflectable post 404 may be formed separatelyfrom, and securely attached to, one or more of mount 414 and retainer402 by laser welding, soldering or other bonding technology.Alternatively, deflectable post 404 may be formed separately andmechanically engage one or more of mount 414 and retainer 402 using, forexample, threads, a lock ring, toothed locking washer, cotter pin orother mechanism.

Sleeve 406 is made of a compliant material which permits movement ofdeflectable post 404 relative to shield 408. The sleeve 406 controlsdeflection of the deflectable post 404. Sleeve 406 is preferably made ofa compliant biocompatible polymer. The properties of the material anddimensions of the sleeve 406 and deflectable post 404 are selected toachieve the desired force/deflection characteristics for deflectablepost 404. In a preferred embodiment, the sleeve is made of PCU, is 2 mmthick when uncompressed and may be compressed to about 1 mm in thicknessby deflection of the post. Sleeve 406 may also be shaped to modify thecompliance of sleeve 406, for example by providing flutes 407 (notshown). Sleeve 406 fits inside shield 408 surrounding deflectable post404.

Deflection rod 400 is configured to be mounted in a bone anchor 420,which comprises a bone screw 422 connected to a housing 430. Housing 430has a short cavity 432 oriented along the axis of bone anchor 420 at theproximal end and configured to receive the threaded distal end of shield408. Shield 408 also has an outer surface 434 adapted for mounting anoffset connector. Outer surface 434 may, in some embodiments, becylindrical or may have surface features, for example flutes aspreviously discussed. Note that in this embodiment, the deflection rodis arranged coaxially with bone anchor 422. In particular, post 404 ofdeflation rod 400 is arranged coaxially with bone anchor 422.

Referring now to FIG. 4B, which shows a perspective view of a fullyassembled deflection rod 400. When assembled, deflectable post 404 ispositioned within sleeve 406; sleeve 406 is positioned within shield408. Retainer 402 of FIG. 4A and the threaded distal end of shield 406(see FIG. 4A) are then placed in short cavity 432 (not shown) of boneanchor 420. Shield 408 is tightened to housing 430 using the threads andmay also be laser welded to further secure shield 408 to housing 430.Threaded collar 410 is then secured in the threaded proximal end ofshield 408. Threaded collar 410 has two sockets 411 for receiving thepins of a pin wrench to allow threaded collar 410 to be tightened tothreads 438 (see FIG. 4A) of shield 408. Threaded collar 410 may also belaser welded to shield 408 after installation to further secure thecomponents.

FIG. 4C shows a sectional view of deflection rod 400 assembled with abone anchor 420 along the axis indicated by line C-C of FIG. 4B. Asshown in FIG. 4C, ball-shaped retainer 402 fits into a hemisphericalpocket 439 inside housing 430. Shield 408 includes a flange 409 whichholds ball-shaped retainer 402 within hemispherical pocket 439. Collar410 secures sleeve 406 within shield 408. Collar 410 also provides alimit surface 413 for limiting deflection of deflectable post 404.Sleeve 406 occupies the space between deflectable post 404 and shield408 and is compressed by deflection of deflectable post 404 towardsshield 408 in any direction.

FIG. 4D illustrates the deflection of deflectable post 404. Applying aforce to mount 414 causes deflection of deflectable post 404 ofdeflection rod 400. Initially deflectable post 404 pivots about a pivotpoint 403 indicated by an X. Deflectable post 404 may pivot in anydirection about pivot point 403. In this embodiment, pivot point 403 islocated at the center of ball-shaped retainer 402. Pivot point 403 maybe positioned closer to the distal end of bone screw 422, for example byprojecting a virtual pivot point. As shown in FIG. 4D, deflection ofdeflectable post 404 initially compresses the material of sleeve 406.The force required to deflect deflectable post 404 depends upon thedimensions of deflectable post 404, sleeve 406 and shield 408 as well asthe attributes of the material of sleeve 406. After further deflection,deflectable post 404 comes into contact with limit surface 413 of collar410.

Limit surface 413 is shaped and oriented to reduce the possibility ofwear and damage to deflectable post 404 due to contact with limitsurface 413. For example, limit surface 413 is oriented such that whendeflectable post 404 makes contact with limit surface 413, the contactis distributed over an area to reduce wear and stress on deflectablepost 404. After deflectable post 404 comes into contact with limitsurface 413, further deflection requires deformation of deflectable post404. Because deflectable post 404 is relatively stiff, the forcerequired to deflect deflectable post 404 will increase significantlyafter contact of deflectable post 404 with collar 410. In a preferredembodiment, deflectable post 404 may deflect from 0.5 mm to 2 mm in anydirection before making contact with limit surface 413. More preferably,deflectable post 404 may deflect approximately 1 mm before makingcontact with limit surface 413.

As depicted in FIG. 4D, when load or force is first applied to thedeflection rod by the spine, the deflection of the deflection rodresponds about linearly to the increase in the load during the phasewhen deflection of deflectable post 404 causes compression of sleeve406. After about 1 mm of deflection, when deflectable post 404 contactslimit surface 413 the deflection rod becomes stiffer. A greater amountof load or force needs to be placed on the deflection rod in order toobtain the same amount of deflection that was realized prior to thispoint because further deflection requires bending of deflectable post404. The relationship between deflection and load is thus a non-linearfunction. Accordingly, the deflection rod of this example provides arange of motion where the load supported increases about linearly as thedeflection increases and then with increased deflection the loadsupported increases more rapidly in a non-linear manner in order toprovide dynamic stabilization.

FIG. 4E shows a modified version 400 e of deflection rod 400. All partsare the same with the exception that deflectable post 404 e is modifiedto receive a ball 444. Ball 444 is received in a pocket 442 at one endof vertical rod 450. Ball 444 is secured within pocket 442 by cap 446.Ball 444 forms part of ball joint 440 which connects deflectable post404 e to vertical rod 450. Ball joint 440 allows greater range of motionand reduces torsional stresses on the dynamic stabilization assembly andthe bones to which it is attached. However, using a ball-joint in placeof a fixed connection between the vertical rod and post also reduces thestiffness of the assembly. Thus the choice of ball-joint or fixedconnection is a trade-off between stiffness and range of motion. Thechoice will be made based on the patient's anatomy and functionalrequirements.

Deflection rod 400 and bone anchor 420 may be used in the same manner aspreviously described. Deflection rod 400 may be utilized, for example,to provide load sharing and dynamic stabilization. Deflection rod 400may be connected to another level of a spine using a vertical rodconnected to a polyaxial screw at that level or another deflection rodand bone anchor. By utilizing offset connectors mounted to the exteriorsurface of shield 408, deflection rod 400 can connect to both adjacentvertebrae thereby providing the ability to construct a multi-leveldynamic spinal stabilization assembly suitable for providing loadsharing and stabilization.

FIG. 4F shows a sectional view of an alternative embodiment of adeflection rod 460. As shown in FIG. 4F post 464 includes a retainer 462which is engaged by sleeve 468 to secure post 464 to bone screw 461. Indeflection rod 460, retainer 462 is a cone-shaped portion of a sphereinstead of spherical in shape. Retainer 462 pivots about pivot point 463marked by an X. Dotted line 465 shows the diameter of a sphericalretainer having the same effective pivot point. The conical retainer 462requires a lower volume while providing an effective pivot point closerto the distal end of bone screw 461. The conical retainer can beaccommodated in a smaller cavity within bone screw 461. This allows bonescrew 461 to be stronger while allowing pivot point 463 to be placedcloser to the instantaneous center of rotation of the spine. Placing thepivot point 463 closer to the instantaneous center of rotation of thespine also helps isolate the bone anchor from the relative motionbetween the vertebrae. When implanted, pivot point 463 is preferablypositioned at or below the surface of the vertebra. More preferably,pivot point 463 is positioned within the pedicle or vertebral body of avertebra. This retainer may be substituted for the retainers in theother deflection rods described herein.

FIG. 4G shows a sectional view of another alternative embodiment of adeflection rod 470. As shown in FIG. 4G post 474 includes a retainer 472which is engaged by sleeve 478 to secure post 474 to bone screw 471. Indeflection rod 470, retainer 472 is segment of the surface of a sphereinstead of being spherical. Retainer 472 pivots about pivot point 473marked by an X. Dotted line 475 shows the diameter of a sphericalretainer having the same effective pivot point. The center of the sphere(if complete) is outside of retainer 472. However, post 474 behaves asif pivoting about this virtual pivot point (so-called because it isexternal to the pivot mechanism). Retainer 472 requires a lower volumethan a spherical retainer. Retainer 472 can therefore be accommodated ina smaller cavity within bone screw 471. This allows bone screw 471 to bestronger while allowing pivot point 473 to be placed closer to theinstantaneous center of rotation of the spine. Placing the pivot point473 closer to the instantaneous center of rotation of the spine alsohelps isolate the bone anchor from the relative motion between thevertebrae. When implanted, pivot point 473 is preferably positioned ator below the surface of the vertebra. More preferably pivot point 473 ispositioned within the pedicle or vertebral body of a vertebra. Thisretainer may be substituted for the retainers in the other deflectionrods described herein.

Deflection/Load Response Curve

As previously stated, the deflection response of a deflection rod can becustomized based on the choice of design, dimensions and materials. Itis contemplated, for example, that the deflection rod can be made instiffness that can replicate a 70% range of motion and flexibility ofthe natural intact spine, a 50% range of motion and flexibility of thenatural intact spine and a 30% range of motion and flexibility of thenatural intact spine for providing in a kit for a doctor to use. FIG. 5Ais a graph showing the deflection/force response of three differentdeflection rod assemblies.

Curve 501 shows the force/deflection response of a deflection rod thatis a little stiffer than a 70% stiffness deflection rod. The deflectionrod 504 tested to generate the force/deflection curve 501 shown in FIG.5A is shown schematically in FIG. 5B. Deflection rod 504 has a PEEKsleeve 512 of about 4 mm in diameter at its largest diameter and adeflectable post 510 made of Nitinol. Sleeve 512 and deflectable post510 are press fit inside a sleeve 516. There is a gap 508 between thesleeve 512 and shield 516 which allows initial deflection of deflectablepost 510 without compression of sleeve 512. The working length of thedeflection rod 504 is about 26 mm. The deflectable post 510 is connectedby a ball-joint 522 to the vertical rod 520. The deflection of thedeflectable post 510 post in response to load applied on the verticalrod 522 is shown by the curve 501. As is evident from FIG. 5A, thedeflection/force curve 501 is non-linear. At about 1 mm of deflection,sleeve 512 makes contact with the shield 516 and further deflectionrequires compressions of sleeve 512 as well as bending of thedeflectable post 510. The deflection rod 504 therefore responds morestiffly as the load increases. As the deflection increases, thestiffness of the deflection rod increases such that the force requiredper unit of additional deflection increases in response to the loadplaced on the spine and deflection rod. This can be observed in theforce/deflection curve as an increase in the slope of the curve 501.

Curve 502 shows the force/deflection response of an alternativedeflection rod. The deflection rod tested to generate theforce/deflection curve 502 shown in FIG. 5A is of the same generaldesign as deflection rod 300 of FIGS. 3A-3C. The deflection rod has a 2mm thick sleeve of Bionate® PCU having a durometer of 80. Thedeflectable post is made of titanium and varies between 4 mm and 5 mm indiameter. The length of the deflectable post (including retaining balland ball joint) is about 20 mm. The deflection rod is connected by aball-joint to the vertical rod. The deflection of the deflectable postin response to load applied on the vertical rod is shown by the curve502. As is evident from curve 502 the deflection rod responds graduallymore stiffly as the load increases. Curve 502 was obtained without acollar and limit surface. In deflection rods made according to thedesigns illustrated in FIGS. 3A-3H and 4A-4D, the stiffness of thedeflection rod should increase at about 1 mm of deflection thedeflectable post makes contact with the limit surface. This isillustrated in the predicted force/deflection curve 502 b as a suddenincrease in slope of the curve. Thus, as the deflection increases, thestiffness of the deflection rod increases such that the force requiredper unit of additional deflection increases rapidly in response to theload placed on the spine and deflection rod.

Curve 503 shows the force/deflection response of an alternativedeflection rod. The deflection rod tested to generate theforce/deflection curve 502 shown in FIG. 5A is of the same generaldesign as deflection rod 300 of FIGS. 3A-3C. The deflection rod testedto generate the force/deflection curve 503 shown in FIG. 5A also has a 2mm thick sleeve of Bionate® PCU having a durometer of 80. Thedeflectable post is made of titanium and varies between 4 mm and 5 mm indiameter. The length of the deflectable post (including retaining balland mount) was about 20 mm. The deflection rod is, however, connected bya rigid connection between the mount and the vertical rod (instead of aball-joint). The deflection of the post in response to load applied onthe vertical rod is shown by the curve 503. As is evident from curve 503the deflection rod responds much more stiffly than when connected via aball-joint. Curve 503 was also obtained without a collar and limitsurface. In deflection rods made according to the designs illustrated inFIGS. 3A-3H and 4A-4D, the stiffness of the deflection rod shouldincrease further at about 1 mm of deflection when the post makes contactwith the limit surface. This is illustrated in the predictedforce/deflection curve 503 b as a sudden increase in the slope of thecurve. Thus, as the deflection increases, the stiffness of thedeflection rod increases such that the force required per unit ofadditional deflection increases rapidly in response to the load placedon the spine and deflection rod.

As depicted in FIG. 5A, as load or force is first applied to thedeflection rod by the spine, the deflection of the deflection rodresponds about linearly to the increase in the load. After the postmakes contact with the limit surface, the deflection rod responds morestiffly. In this region, a greater amount of load or force needs to beplaced on the deflection rod in order to obtain the same amount ofdeflection that was realized prior to this point. Accordingly, thedeflection rod of this example provides a range of motion where the loadsupported increases about linearly as the deflection increases and thenwith increased deflection the load supported increases more rapidly in anon-linear manner. The transition from lower stiffness to higherstiffness region depends upon the distance between the deflectable postand the limit surface of the collar. This distance may be customized aspreviously described so that the transition occurs after the desiredamount of deflection, for example after about 1 mm of deflection orafter about 2 mm of deflection.

Deflection Rods Having Anisotropic Deflection Characteristics

The deflection system of the present invention provides in someembodiments the ability to control stiffness for extension, flexion,lateral bending and axial rotation, and to control stiffness for each ofthese motions independently of the other motions. The characteristics ofthe deflection rod can be changed, for example, by adjusting thediameter of post and/or the thickness of the sleeve and/or the distancebetween the post and the limit surface. For example, FIGS. 6A-6D showcross-sections through alternative embodiments of the deflection rod 200of FIG. 2B along the line D-D. Similar variations could be made of theother deflection rods described herein e.g. deflection rod 300 of FIGS.3A-3C and deflection rod 400 of FIGS. 4A-4C.

In FIG. 6A, the shield 208 a is uniformly thicker and sleeve 206 a ofdeflection rod 200 a is uniformly thinner than that of deflection rod200. As a consequence, deflection post 204 a with the cross-sectionshown in FIG. 6A is stiffer/less flexible in all directions.

However, the deflection characteristics need not be isotropic. A biascan be introduced in the deflection rod by varying the inner surface ofthe shield and/or the thickness of the sleeve in different positionsaround the post. As shown in FIG. 6B, shield 208 b is designed such thatthe sleeve 206 b of deflection rod 200 b is thicker on the left andright sides of deflectable post 204 b than in the top and bottom sides(relative to the page). A deflection rod 200 b having the cross-sectionshown in FIG. 6B is thus stiffer in the up and down directions and moreflexible in the left and right directions.

As shown in FIG. 6C, shield 208 c is designed so that sleeve 206 c ofdeflection rod 200 c is thicker on the right, top, and bottom sides ofdeflectable post 204 c than on the left side. A deflection rod 200 chaving the cross-section shown in FIG. 6C is thus stiffer in the leftdirection and more flexible in the right, up and down directions.

As shown in FIG. 6D, shield 208 d is designed so that sleeve 206 d ofdeflection rod 200 d is thicker on the right side of deflectable post204 d than on the left, top, or bottom sides. Deflection rod 200 dhaving the cross-section shown in FIG. 6D is, thus, more flexible in theright direction than in any of the other directions.

The characteristics of the deflection rod can also be changed by, forexample, adjusting the compliance of the sleeve. For example, using PCUwith different durometer ratings and or using different materials.Making the entire sleeve of a less compressible material for examplewill make the deflection rod stiffer, e.g. a larger load will berequired to cause the same deflection. Conversely, making the entiresleeve of a more compressible material for example will make thedeflection rod more flexible, e.g. a smaller load will be required tocause the same deflection. However, the deflection characteristics neednot be isotropic. A bias can be introduced in the deflection rod 200 byhaving material with different compliance in different regions of thesleeve. This can be achieved, for example, using a multi-shot injectionmolding technique to make the sleeve and using PCUs having differentdurometer ratings in the multi-shot injection process. For example,FIGS. 6E-6H show cross-sections through alternative embodiments of thedeflection rod 200 of FIG. 2B along the line D-D in which the sleeveshave regions with different properties.

As shown in FIG. 6E, sleeve 206 e of deflection rod 200 e has a regionof more compliant material 640 on the left side of deflectable post 204e and a region of less compliant material 680 on the right side(relative to the page). Accordingly, material 640 can have a differentdurometer value than material 680. Both can be a polymer. A deflectionrod 200 e having the cross-section shown in FIG. 6E is thus stiffer inthe right direction and more flexible in the left direction.

As shown in FIG. 6F, sleeve 206 f of deflection rod 200 f has a regionof more compliant material 640 on the top and bottom sides ofdeflectable post 204 f and a region of less compliant material 680 onthe left and right sides (relative to the page). A deflection rod 200 fhaving the cross-section shown in FIG. 6E is thus stiffer in the leftand right directions and more flexible in the up and down directions.

As shown in FIG. 6G, sleeve 206 g of deflection rod 200 g has a regionof more compliant material 640 on the left and bottom sides ofdeflectable post 204 g and a region of less compliant material 680 onthe top and right sides (relative to the page). A deflection rod 200 ghaving the cross-section shown in FIG. 6G is thus stiffer in the up andright directions and more flexible in the down and left directions.

As shown in FIG. 6H, sleeve 206 h of deflection rod 200 h has a regionof more compliant material 640 on the left, top, and bottom sides ofdeflectable post 204 h and a region of less compliant material 680 onthe right side (relative to the page). A deflection rod 200 h having thecross-section shown in FIG. 6H is thus stiffer in the right directionand more flexible in the other directions.

Furthermore, by varying the shape of the collar, the distance betweenthe post and the limit surface of the collar may also be varied. Bymaking the distance shorter, the amount of deflection can be reducedthat occurs before the dramatic increase in stiffness caused by contactwith the limit surface. The collar may be shaped to reduce the gapbetween the post and the limit surface uniformly or may be shaped toreduce the gap between the post and the limit surface more in somedirections than in others (anisotropically).

In embodiments where the deflection rod has anisotropic force-deflectionresponse, it is important to ensure that the deflection rod is implantedin the correct orientation. The deflection rod is therefore providedwith discernable visual or physical characteristics (e.g. an arrow,color, indentation or other observable indicator) which guide thesurgeon to the correct orientation of implantation. When correctlyinstalled, a deflection rod with anisotropic force-deflection responsemay be used to control stiffness for extension, flexion, lateral bendingand axial rotation independently. For example, if a deflection rod ismore flexible in the upward direction (relative to the spine afterimplantation—the head direction being up), the post can deflect morewhen the spine is placed in flexion and can deflect less when the spineis placed in extension. In effect, this arrangement is more restrictivewith respect to movement of the spine with the spine in extension andless restrictive with respect to the movement of the spine with thespine in flexion. Conversely, if the deflection rod is more compliant inthe down direction (relative to the spine after implantation—the headdirection being up), the post can deflect more when the spine is placedin extension and can deflect less when the spine is placed in flexion.In effect, this arrangement is more restrictive with respect to movementof the spine in flexion and less restrictive with respect to themovement of the spine in extension.

Alternative Bone Anchors

FIGS. 7A through 7H illustrate some possible variations in bone anchorsof the anchoring system. The bone anchors each have a housing compatiblewith the deflection rods of the deflection system and the offsetheads/connectors of the connector system. In some embodiments, the boneanchors are installed prior to implantation of the bone anchors in thebody. In alternative embodiments, the bone anchors may be implanted inthe body before installation of a deflection rod.

Bone anchor 710 of FIG. 7A is a bone screw having a threaded region 714which extends up over most of a housing 712. A deflection rod 704 isinstalled in housing 712. The threaded region 714 may extend over agreater or lesser amount of housing 712 depending upon such factors asthe length of the bone screw, the type of bone in which the screw is tobe implanted and the desired height to which the housing 712 will extendabove the bone surface after implantation. Bone anchor 710 may be usefulto lower the depth of the pivot point of the deflection rod 704 closerto the natural instantaneous center of rotation of the spine. Note alsothat the distal thread depth 716 may be deeper than the proximal threaddepth 718. The distal thread depth 716 is adapted for engagement of thesoft cancellous bone while the proximal thread depth 718 is adapted forengagement of the harder cortical bone at the surface of the vertebra.

Bone anchor 720 of FIG. 7B is a bone screw in which the screw-onlysection 724 is shorter in length than in bone screw 710 of FIG. 7A. Adeflection rod 704 is installed in housing 722. Different lengths ofscrew-only section may be useful in different patients or differentvertebrae as the size of the bone in which the anchor needs be implantedmay vary considerably. For example, short bone screws are desirablewhere the dynamic stabilization system is to be implanted in smallervertebrae. The physician may determine the length of bone screwappropriate for a particular patient by taking measurements during theprocedure of by determining measurements from non-invasive scanning, forexample, X-ray NMR, and CT scanning. Note however, that housing 722 ispreferably the same size and shape as the housings of the other boneanchors so as to be compatible with the same deflection rods andconnectors.

Bone anchor 730 of FIG. 7C is a bone screw in which the screw-onlysection 734 has a smaller diameter and is shorter in length than in bonescrew 710 of FIG. 7A. A deflection rod 704 is installed in housing 732.Different diameters of screw-only section may be useful in differentpatients or different vertebrae as the size of the bone in which theanchor needs be implanted may vary considerably. For example, smallerdiameter bone screws may be desirable where the dynamic stabilizationsystem is to be implanted in smaller vertebrae. The physician maydetermine the diameter of bone screw appropriate for a particularpatient by taking measurements during the procedure by determiningmeasurements from non-invasive scanning, for example, X-ray NMR, and CTscanning. Note however, that housing 732 is preferably the same size andshape as the housings of the other bone anchors to be compatible withthe same deflection rods and connectors.

Bone anchor 740 of FIG. 7D is a bone screw in which the housing 742 hasa rim 744 extending away from housing 742 where it transitions to thethreaded region 746. A deflection rod 704 is installed in housing 742.Rim 744 may serve to retain an offset head mounted to housing 742 in away that it can rotate freely around housing 742 during installation.Rim 744 may also serve to widen the contact area between the bone anchor740 where it meets the bone of the vertebra. This can act as a stoppreventing over-insertion. This can also provide a wide base forstabilizing the housing against lateral motion and torque. Note thathousing 742 is preferably the same size and shape as the housings of theother bone anchors so as to be compatible with the same deflection rodsand connectors.

Bone anchor 750 of FIG. 7E illustrates a bone hook device 751 having ahousing 752. A deflection rod 704 is installed in housing 752. Bone hookdevice 751 comprises a bar 754 to which housing 752 is rigidlyconnected. At either end of bar 754 is a bone hook 756 having a setscrew 757 for securing the bone hook 756 to the bar 754. Each bone hook756 has a plurality of sharp points 758 for engaging and securing thebone hook 756 to a vertebra. During use, the bone hooks 756 are urgedtowards each other until the sharp points engage and/or penetrate thesurface of a bone. Set screws 757 are tightened to secure bone hooks 756in position relative to bar 754 and thus secure housing 752 relative tothe bone. Different arrangements of bone hooks and bars may be madesuitable for attachment of the housing 752 to different types, sizes,shapes and locations of vertebra. Note that housing 752 is preferablythe same size and shape as the housings of the other bone anchors so asto be compatible with the same deflection rods and connectors.

In some embodiments, the bone anchors may be provided with atorque-limiting and/or breakaway head which is engaged by a driver todrive the bone anchor into the vertebra. The torque-limiting and/orbreakaway head is designed to prevent further driving of the bone anchorinto the vertebra when the torque applied by the driver exceeds apredetermined torque limit. In preferred embodiments, the torque limitis selected so that the torque required to drive the bone anchor into avertebra is lower than the torque limit. In preferred embodiments, whenthe bone anchor is fully implanted in the vertebra, further rotation ofbone anchor requires significantly higher torque which is higher thanthe torque limit. Thus, when bone anchor is fully implanted in thevertebra, the driver torque exceeds the torque limit and torque-limitingand/or breakaway head prevents the bone anchor from being driven furtherinto the bone. In some cases, the torque limit may be reached prior tocomplete installation of the bone anchor. In such cases, the bone anchormay be removed and a new bone anchor installed—the cavity in the bone inwhich the bone anchor is to be installed may be enlarged to facilitateimplantation of the second bone anchor thereby reducing the torquenecessary to implant the bone anchor.

Bone anchor 760 of FIGS. 7F-7H is a bone screw having a head 770extending beyond a housing 772. Housing 772 is preferably the same sizeand shape as the housings of the other bone anchors so as to becompatible with the same deflection rods and connectors. Head 770 is atorque-limiting head which is designed to break away from bone anchor760 when the torque applied to head 770 exceeds a predetermined torquelimit. Head 770 may be a hex head (as shown) or may be of another designsuitable for being driven by an installation tool for example a wrenchor other driver, including, for example, slotted, Phillips, square,Allen, and Torx heads. During installation, a driver (for example a hexsocket) engages head 770 and rotates bone anchor 760 to drive threadedregion 764 into a vertebra. In preferred embodiments, the torque limitis selected so that the torque required to drive threaded region 764into a vertebra is lower than the torque limit. In preferredembodiments, when the threaded region 764 is fully implanted in thevertebra, further rotation of bone anchor 760 requires significantlyhigher torque which is higher than the torque limit. Thus, when thethreaded region 764 is fully implanted in the vertebra, the drivertorque exceeds the torque limit and the head 770 breaks away from boneanchor 760 as shown in FIG. 7G.

As shown in FIG. 7G, when the torque limit is exceeded, head 770 breaksaway from bone anchor 760. Head 770 may then be removed from cavity 766of bone anchor 760. Cavity 766 may contain internal features 768designed to engage a driver to permit removal of bone anchor 760, ifnecessary or desired. Alternatively or additionally, the exteriorsurface 763 of housing 762 may be provided with features for exampleknurling or splines (not shown but see, e.g., FIG. 7A) which allow adriver to engage the external surface 763 of housing 762 to permitremoval of bone anchor 760 if necessary or desired. As shown in FIG. 7G,head 770 comprises a shaft 772 which extends into cavity 766 of housing762. When head 770 has been removed, cavity 766 is open, to receive adeflection system component or connection system component as previouslydescribed (not shown).

FIG. 7H shows an enlarged sectional view of bone anchor 760 through head770. As shown in FIG. 7H, head 770 is connected by shaft 772 to boneanchor 760. Shaft 772 has a neck 774 of smaller diameter than theremainder of shaft 772. The neck 774 is therefore subjected to higherstress than the remainder of shaft 772. The diameter and material ofneck 774 is selected to control the torque limit. For a particularmaterial, reducing the diameter of the neck reduces the maximum torquewhich can be transmitted by the head before the neck shears off. Whenthe torque limit is exceeded, the shaft 772 shears off at the neck 774.Neck 774 is positioned so that, when neck 774 is sheared, any portion ofshaft 772 which remains attached to bone anchor 760 is positioned so asnot to obstruct installation of a deflection system component orconnection system component within cavity 766. As shown in FIG. 7H, forexample, neck 774 may be positioned within a depression 767 in thedistal end of cavity 766. When neck 774 shears off, cavity 766 is clearfor installation of another component. In some cases, head 770 and shaft772 may be formed in one piece with bone anchor 760. In otherembodiments, head 770 and shaft 772 may be formed separately from boneanchor 760 and subsequently securely connected to bone anchor 760 by abonded, welded or mechanical joint.

Alternative Deflection Rods/Loading Rods

One feature of embodiments of the present invention is load sharingprovided by the deflection rod. The deflection rod provides stiffnessand support where needed to support the loads exerted on the spineduring normal spine motion thereby recovering improved spine functionwithout sacrificing all motion. The deflection rod also isolates theanchor system components from forces exerted by the dynamicstabilization assembly thereby reducing stress on the bone anchors andthe bone to which they are attached. In particular embodiments, thedeflection rods of the present invention are oriented coaxial with thelongitudinal axis of the bone anchor to which they are attached or inwhich they are incorporated. Moreover, by selecting the appropriatestiffness of the deflection rod or loading rod to match the physiologyof the patient and the loads that the patient places on the spine, abetter outcome is realized for the patient.

FIGS. 8A-8H shows alternative deflection rods having differentmechanisms to secure the deflectable post to the deflection rod and/orthe bone anchor. The mechanisms of FIGS. 8A-8H may be adapted for use inother of the deflection rods described herein. For example, FIGS. 8A-8Cillustrate one alternative embodiment of a deflection rod/loading rod.Deflection rod 800 of FIGS. 8A-8C is similar in design and function todeflection rod 300 of FIGS. 3A-3E. However, deflection rod 800 utilizesa different mechanism to secure the ball-shaped retainer to the housingof the bone anchor than is used in deflection rod 300.

FIG. 8A shows an exploded view of alternative deflection rod 800.Deflection rod 800 includes ball-shaped retainer 802, post 804, sleeve806, locking pin 808, collar 810, and mount 814. In this embodiment,ball-shaped retainer 802 is formed in one piece with post 804.Ball-shaped retainer 802 is split along the longitudinal axis of post804 by one or more slots 803. Slots 803 allow ball-shaped-retainer 802to deform to have a reduced diameter. A shaft 805 passes from theproximal end of mount 814 through post 804 and communicates with the oneor more slots 803. A locking pin 808 may be inserted through shaft 805to occupy space in the one or more slots 803. With locking pin 808secured in place, ball-shaped retainer 802 is locked at its normaldiameter and may not be compressed to a smaller diameter.

Sleeve 806 fits inside cavity 832 of housing 830 surrounding post 804.Sleeve 806 is made of a compliant material which permits movement ofpost 804 relative to housing 830. Deflection rod 800 is configured to bemounted in a bone anchor 820, which comprises a bone screw 822 connectedto a housing 830. Housing 830 has a cavity 832 oriented along the axisof bone anchor 820 at the proximal end and configured to receivedeflection rod 800. Housing 830 also has an outer surface 834 adaptedfor mounting a component, e.g. an offset connector. As shown in FIG. 8A,outer surface 834 of housing 830 is provided with flutes 836. Flutes 836may be engaged by a driver for implanting bone anchor 820.

Referring now to FIG. 8B, which shows a perspective view of a deflectionrod 800 assembled with a bone anchor 820. When assembled, deflectablepost 804 is positioned within sleeve 806 of FIG. 8A. Post 804 and sleeve806 are then placed in the cavity 832 of bone anchor 820 (See FIG. 8A).Locking pin 808 is then inserted into shaft 805 (not shown) to secureball-shaped retainer 802 to bone anchor 820. Locking pin 808 may also belaser welded to mount 814 after installation to further secure thecomponents. Threaded collar 810 is then secured in the threaded proximalend of cavity 832. Threaded collar 810 has two sockets 811 for receivingthe pins of a pin wrench to allow threaded collar 810 to be tightened tothreads 838 of housing 830. Threaded collar 810 is laser welded tohousing 830 after installation to further secure the components.Threaded collar 810 secures sleeve 806 within cavity 832 of bone anchor820.

FIG. 8C shows a sectional view of a deflection rod 800 assembled with abone anchor 820 along the axis indicated by line C-C of FIG. 8B. Asshown in FIG. 8C, sleeve 806 occupies the space between post 804 andhousing 830. Sleeve 806 is compressed by deflection of post 804 towardshousing 830 in any direction. Ball-shaped retainer 802 fits into apocket 839 in the bottom of cavity 832 (not shown) of housing 830.Pocket 839 has the shape of a major spherical cap (a spherical capcomprising greater than half of the sphere). Consequently, the entranceaperture 840 to pocket 839 is narrower than the major diameter of pocket839. Ball-shaped retainer 802 has the same diameter as the majordiameter of pocket 839; however, in the absence of locking pin 808,ball-shaped retainer 802 may be compressed sufficiently to pass throughaperture 840. However, after ball-shaped retainer has been pushed intopocket 839 and locking pin 808 has been installed in shaft 805 and slot803, ball shaped retainer 802 can no longer be compressed and istherefore locked into pocket 839 while still allowing rotation ofball-shaped retainer 802. Collar 810 secures sleeve 806 within housing830. The deflection rod 800 of FIG. 8A-8C does not include a shieldbetween sleeve 806 and housing 830. By removing the thickness of theshield, the size/strength properties of the device may be enhanced.

As shown in FIG. 8D, an alternative deflection rod 850 may utilizeretainer 852 which is a portion of a sphere rather than spherical. Inother aspects, the deflection rod 850 is similar to deflection rod 800of FIGS. 8A-8C. However, a full sphere may be unnecessary for retainer852 where, as here, the post 804 is limited by collar 810 to a fewdegrees of deflection. Over the range of deflection permitted, thespherical segment retainer 852 maintains sufficient contact with thewall of pocket 854 to secure retainer 852 within pocket 854. The lowerportion 851 of retainer 852 is, in this embodiment, a sphere having asmaller diameter than the upper portion, but having the same center ofrotation. Pocket 852 can likewise be reduced in size by truncating themajor spherical cap to form a pocket 854 in the shape of a sphericalsegment (which includes the center and thus maximum diameter of thesphere). A smaller lower pocket 853 of the same diameter as the lowersphere serves to locate the retainer during its travel. As a consequenceless material needs to be removed to form pocket 854 and the strength ofbone anchor 850 is therefore enhanced. Other shapes of retainer andpocket may also be used so long as they secure post 804 to bone anchor820 and allow the desired range of travel for post 804.

FIGS. 8E and 8F, show exploded and sectional views of an alternativedeflection rod 860 which uses a retainer in the form of a split-ring862. Split spherical ring 862 fits in pocket 864 in bone anchor 861.Pocket 864 has the shape of a major spherical cap (a spherical capcomprising greater than half of the sphere). Consequently, the entranceaperture 865 to pocket 864 is narrower than the major diameter of pocket864. Split-ring 862 has the same diameter as the major diameter ofpocket 864. However, split-ring retainer 862 may be compressedsufficiently to pass through aperture 865. After split-ring retainer 862has been pushed into pocket 864, post 866 is pushed into the centralaperture of split-ring retainer 862. With post 866 locked into thecentral aperture of split-ring retainer 862, split-ring retainer 862 canno longer be compressed and is therefore locked into pocket 864 whilestill allowing rotation of retainer 862. Post 866 can be secured tosplit-ring retainer 862 with an internal split-ring 868. Other fittingssuch as threads, clips and or tabs may also be utilized to secure post866 to split-ring retainer 862. Alternatively bonding technology may beused to secure post 866 to split-ring retainer 862, e.g. laser welding.

Deflection rod 860 may be provided with a collar similar to collar 810of FIGS. 8A-8C. However, FIGS. 8E and 8F show an alternative embodimentin which post 866 includes a flange 870. Flange 870 is shaped to remainin close proximity to the top surface of housing 871 of bone anchor 861as post 866 pivots. Flange 870 retains sleeve 806 within housing 871without need of a collar. The interior surface of housing 871 is shapedto provide the limit surface 872 to limit deflection of post 866. Byremoving the thickness of the shield and the need for a separate collar,the size/strength properties of the device may be enhanced.

A ball may be locked in a ball-joint pocket in a variety of ways. Somesuitable methods and devices for locking a ball in a ball-joint assemblyare disclosed in U.S. Pat. No. 4,666,330 titled “Ball Joint Assembly” toO'Connell et al. which is incorporated herein by reference in itsentirety. FIGS. 8G and 8H, show exploded and sectional views of analternative deflection rod 880 which uses a ball-shaped retainer 882.Ball-shaped retainer 882 fits in pocket 884 in bone anchor 881. Pocket884 is hemispherical. The entrance aperture 885 to pocket 884 is thesame diameter as ball-shaped retainer 882. However, entrance aperture885 includes a groove 883 which receives a split-ring 888. Split-ring888 has a larger diameter than aperture 885 but split-ring 888 iscompressed slightly during installation. After passing through aperture885, split-ring 888 expands outwards to occupy groove 883. Split-ring888, when positioned in groove 883, reduces the effective diameter ofaperture 885 and prevents removal of ball-shaped retainer 882. Sleeve806 is then inserted in housing 891 of bone anchor 881. Collar 890secures sleeve 806 within housing 891 of bone anchor 881. By removingthe thickness of the shield, the size/strength properties of the devicemay be enhanced.

In some embodiments, as described above, the deflection rod includes adeflectable post, an outer sleeve, and a mount which includes a shieldpositioned around the sleeve. The movement of the deflectable postrelative to the mount allows controlled movement of the bone anchor (andvertebra in which it is implanted) relative to the vertical rods therebysupporting the vertebrae to which the bone anchors are attached whileallowing movement of the vertebrae. However, as illustrated in FIGS.9A-9H controlled movement of the deflectable post (or other connectionpoint for vertical rods) relative to a bone anchor may be achieved usinga number of alternative designs for deflection rods/loading rods. Ingeneral, each mechanism includes a linkage by which deflection of thedeflectable post is tied to compression of a compliant component. Thecompression of the compliant component imparts the deflection rod soformed with the force/deflection characteristics necessary or desirablefor the application.

FIGS. 9A-9C show views of an alternative deflection rod 900 according toan embodiment of the invention. FIG. 9A shows an exploded view of thedeflection rod 900. FIGS. 9B and 9C show sectional views of thedeflection rod 900 with FIG. 9C illustrating deflection of deflectionrod 900 under load. Referring first to FIG. 9A, deflection rod 900 isassembled in cavity 924 of housing 922 of bone anchor 920. A compliantsleeve 906 is first placed into cavity 924. Sleeve 906 is annular andhas a central opening 907. Retaining ring 908 is then inserted intocavity 924. The lower surface of retaining ring 908 is adapted to engagecompliant sleeve 906 and secure it into position. The upper surface ofretaining ring 908 has a pocket 909 adapted to receive the lower portionof a ball 902. Deflectable post 904 is attached to the top of ball 902.A control rod 905 extends from the bottom of ball 902. Control rod 905is shaped to fit through retaining ring 908 into the central opening 907of sleeve 906. With control rod 905 positioned inside sleeve 906 andball 902 in contact with retaining ring 908, a threaded collar 910 istightened into the upper end of cavity 924. The lower surface 911 ofcollar 910 is shaped to form the top half of a pocket in which ball 902may rotate. Collar 910 has sockets for a pin wrench or other featuresallowing the collar to be secured to bone anchor 920. Collar 910 mayalso be bonded or welded into place.

FIG. 9B shows a sectional view of deflection rod 900 when fullyassembled. As shown in FIG. 9B control rod 905 is surrounded by sleeve906. Ball 902 is secured in a pocket formed by retaining ring 908 andcollar 910. Post 904 may pivot in any direction and rotate about itslong axis. However, as shown in FIG. 9C, when post 904 pivots, controlrod 905 also pivots (in the opposite direction) compressing the materialof sleeve 906. Compression of sleeve 906 by control rod 905 imparts thedeflectable post 904 with a controllable force/load response which canbe customized as previously described. A limit surface 912 of collar 910is designed to make contact with deflectable post 910 after apredetermined deflection. Further deflection of the proximal end ofdeflectable post 904 after contact with limit surface 912 requiresbending of deflectable post 904. Thus, the stiffness of deflectable post904 will typically increase dramatically upon contact betweendeflectable post 904 and limit surface 912.

FIGS. 9D-9F show views of an alternative deflection rod 930 according toan embodiment of the invention. FIG. 9D shows an exploded view of thedeflection rod 930. FIGS. 9E and 9F show sectional views of thedeflection rod 930 with FIG. 9F illustrating deflection of deflectionrod 930 under load. Referring first to FIG. 9D, deflection rod 930 isassembled in cavity 954 of housing 952 of bone anchor 950. A compliantdisc 936 is first placed into cavity 954. With compliant disc 936 inposition, deflectable post 934 is then inserted into cavity 954.Deflectable post 934 has a control disc 935 at the distal end. Controldisc 935 fits snuggly against compliant disc 936. Collar 940 is thensecured into the end of cavity 954. The lower surface 941 of collar 940is shaped to form the top portion of a pocket in which control disc 935may pivot and rotate. The edges of control disc 935 and the walls ofcavity 954 are radiussed so that control disc 935 may pivot over thedesired range of travel. Collar 940 may also be bonded or welded intoplace.

FIG. 9E shows a sectional view of deflection rod 930 when fullyassembled. As shown in FIG. 9E control disc 935 sits on top of compliantdisc 936. Control disc 935 is secured in a pocket formed by the walls ofcavity 954 and collar 940. Deflectable post 934 may pivot in anydirection and rotate about its long axis. However, as shown in FIG. 9F,when deflectable post 934 pivots, control disc 935 also pivotscompressing the material of compliant disc 936. Compression of compliantdisc 936 by control disc 935 imparts the deflectable post 934 with acontrollable force/load response which can be customized as previouslydescribed. A limit surface 942 of collar 940 is designed to make contactwith deflectable post 930 after a predetermined deflection. Furtherdeflection of the proximal end of deflectable post 934 after contactwith limit surface 942 requires bending of deflectable post 934. Thusthe stiffness of deflectable rod 930 will typically increasedramatically upon contact between deflectable post 934 and limit surface942.

FIGS. 9G and 9H show views of an alternative deflection rod 960according to an embodiment of the invention. FIG. 9G shows an explodedview of the deflection rod 960. FIG. 9H shows a sectional view of thedeflection rod 960. Referring first to FIG. 9G, deflectable post 964 hasa cavity 965 which receives a portion of bone anchor 980. Deflectablepost 964 also has a threaded mount 963 to which a vertical rod may besecured. A compliant sleeve 966 is first placed into cavity 965.Compliant sleeve 966 has a central opening 967. A retaining ring 968 isthen placed in cavity 965. Retaining ring 968 has a central opening 969.Bone anchor 980 comprises a bone screw 981, a ball 983 and a control rod985. Control rod 985 is passed through central opening 969 of retainingring 968 and into central opening 967 of compliant sleeve 966. Lastly,threaded collar 970 is screwed into the opening 965. Threaded collar 970combines with retaining ring 968 to form a pocket shaped to receive ball983. Ball 983 is secured between retaining ring 968 and collar 970 butcan rotate and pivot relative to deflectable post 964. Collar 970 mayalso be bonded or welded into place.

FIG. 9H shows a sectional view of deflection rod 960 when fullyassembled. As shown in FIG. 9G, compliant sleeve disc 966 sits aroundcontrol rod 985. Deflectable post 964 may pivot in any direction androtate about its long axis. However, when deflectable post 964 pivots,control rod 985 compresses the material of sleeve 966. Compression ofsleeve 966 by control rod 985 imparts the deflectable post 964 with acontrollable force/load response which can be customized as previouslydescribed. A limit surface 972 of collar 970 is designed to make contactwith bone anchor 980 after a predetermined deflection. Furtherdeflection of deflectable post 964 after contact between bone anchor 980and limit surface 972 requires bending of deflectable post 964 or boneanchor 980. Thus the stiffness of deflectable rod 960 will typicallyincrease dramatically upon contact between bone anchor 980 and limitsurface 972.

Alternate Mechanisms for Mounting a Vertical Rod to a Deflection Rod

In order to utilize deflection rods of the present invention toconstruct a dynamic stabilization assembly, the deflection rod iscoupled with a vertical rod. The deflection rod may be coupled to thevertical rods in a fixed, pivoting or flexible manner depending on therequirements of the dynamic stabilization assembly. One mechanism forcoupling a deflection rod to a vertical rod is the ball-joint 222illustrated for example in FIGS. 2A-2C and FIGS. 2E-2G. As shown in FIG.2B, the vertical rod 216 is coupled to the deflectable post 204 by theball-joint 222 in a manner that allows the vertical rod 216 to rotateabout the long axis of the deflectable post 204 and also pivot relativeto the deflectable post 204. These two degrees of freedom are presentboth during implantation and also in the completed dynamic stabilizationassembly. By comparing FIGS. 2C, 2E and 2F, it can be seen that theangle between the vertical rod 216 and deflectable post 204 changes asdeflectable post 204 is deflected. This change in angle is accommodatedby rotation of ball 214 in ball joint 222.

A second mechanism for coupling a deflection rod to a vertical rod isthe threaded mount 314 of deflection rod 300 illustrated in FIGS. 3A-3H.As shown in FIG. 3G, the vertical rod 360 is secured to threaded mount314 by a nut 362. The vertical rod 360 can be rotated around mount 314before nut 362 is tightened but, thereafter, vertical rod 360 is rigidlysecured to deflectable post 304. After completion of the dynamicstabilization assembly, vertical rod can still rotate around the longaxis of bone anchor 320 because deflectable post 304 may rotate relativeto the long axis of bone anchor 320. However, the angle between verticalrod 360 and deflectable post 304 is fixed. Thus, any angle changebetween vertical rod 360 and deflectable post 304 resulting frommovement of the vertebra must be accommodated by deformation (bending)of vertical rod 360 and deflectable post 304. Vertical rod 360 anddeflectable post 304 are relatively stiff and thus, the dynamicstabilization assembly is stiff as compared to a dynamic stabilizationassembly which may accommodate the angle change without bending of thevertical rod and deflectable post using e.g. a ball-joint.

Thus, the mechanism by which the vertical rod is coupled to a deflectionrod affects the ease by which the dynamic stabilization system may beassembled and also the stiffness of the dynamic stabilization assembly.FIGS. 10A-12X show a range of alternative mechanisms for coupling thedeflectable post of a deflection rod to the vertical rod to create thedynamic stabilization assembly.

FIGS. 10A-10E show a hinged coupling 1000 for connecting a deflectionrod to a vertical rod according to an embodiment of the invention.Hinged coupling 1000 is designed to be mounted to the threaded proximalend of a deflectable post of a deflection rod. Hinged coupling 1000 maybe used with and deflection rod having a suitable proximal mount, forexample, deflection rod 300 of FIGS. 3A-3H.

FIG. 10A shows an exploded view of hinged coupling 1000. Hinged coupling1000 comprises clevis 1010, clevis pin 1012, locking plate 1020 and avertical rod 1030. Hinged coupling 1000 is held in place by a nut 1002.Vertical rod 1030 is configured to be received by clevis 1010. One endof vertical rod 1030 includes a disc 1032 having a central aperture1034. The circumference of disc 1032 is provided with locking features,for example, teeth 1036. Disc 1032 is received between arms 1013, 1014of clevis 1010. Pin 1012 is then inserted through aperture 1016 whichpasses through both arms 1013, 1014. Pin 1012 also passes throughcentral aperture 1034 of disc 1032 of vertical rod 1030. Pin 1012 issecured to clevis 1010, either with a mechanical fitting and/or bybonding, for example threads and/or laser welding. With pin 1012 inplace, vertical rod 1030 is secured to clevis 1010 but may pivot aboutthe axis of clevis pin 1012.

Clevis 1010 also has a mounting plate 1017 having an aperture 1018therethrough for receiving the deflectable post of a deflection rod.Aperture 1018 may be circular or may be polygonal (as shown). Whereaperture 1018 is polygonal (non-circular) it may engage a similarlypolygonal post in such a way as to prevent rotation of mounting plate1017 relative to the post. This is advantageous in that such rotationmay cause nut 1002 to be loosened. Moreover, the deflectable of thedeflection rods of the present invention can typically rotate relativeto the bone anchor and thus rotation of the mounting plate 1017 is aredundant and therefore unnecessary degree of freedom. In embodimentswhere the post to which the hinged coupling 1000 is connected may notrotate, it may be desirable to provide a mounting by which mountingplate 1017 may rotate around the post, however in such cases, care mustbe taken to ensure that nut 1002 or such other fastener that is usedsecures the mounting plate in position while allowing such rotation.

FIG. 10B shows hinged coupling 1000 ready for installation on adeflection rod 1001. Deflection rod 1001 has at the end of deflectablepost 1004 a mount 1005 for receiving mounting plate 1016 of hingedcoupling 1000. Mount 1005 has a lip 1006, a polygonal portion 1007 and athreaded portion 1008. Lip 1006 provides a mechanical stop to catchmounting plate 1017. Polygonal portion 1007 fits snuggly in polygonalaperture 1018 to preclude rotation of mounting plate 1017 relative todeflectable post 1004. In use, aperture 1018 is placed over mount 1005until mounting plate 1017 contacts lip 1006 and polygonal portion 1007is received with aperture 1018.

After mounting plate 1017 is positioned, vertical rod 1030 is orientedin the desired direction and angle relative to deflectable post 1004.Typically, vertical rod 1030 is oriented by securing the free end 1031to another device on another vertebra of the spine, e.g. a bone screw,polyaxial screw, or deflection rod. When vertical rod 1030 is correctlypositioned, locking plate 1020 may be installed. Locking plate 1020includes mounting plate 1022 having an aperture 1024 adapted to bereceived over mount 1005 of deflectable post 1004. A locking arm 1026projects from mounting plate 1022. Locking arm 1026 is adapted to fitbetween arms 1013, 1014 of clevis 1010 and engage vertical rod 1030 tosecure vertical rod 1030 at the desired angle with deflection post 1004.Locking arm 1026 is provided with locking features, for example, teeth1028 for engaging the circumference of disc 1032 of vertical rod 1030.In this embodiment, teeth 1028 of locking arm 1026 engage teeth 1036 ofvertical rod 1030 to lock vertical rod 1030 at a fixed angle relative todeflectable post 1004. Clevis 1010 and locking plate 1020 are secured inplace by nut 1002 which engages threaded portion 1008 of mount 1005 tosecure vertical rod 1030 to deflectable post 1004. FIG. 10C showsvertical rod 1030 secured to deflectable post 1004 by hinged coupling1000 with vertical rod 1030 at a fixed angle relative to deflectablepost 1004.

In an alternative mode of installation shown in FIG. 10D, locking plate1020 may be omitted. In such case, nut 1002 is used to secure clevis1010 to mount 1005 of deflectable post 1004. In the absence of lockingplate 1020, vertical rod 1030 is free to pivot about pivot pin 1012 evenafter installation and securing of vertical rod 1030 to deflectable post1004 (see arrow 1038). Thus, vertical rod 1030 is provided with anadditional degree of freedom of motion as finally implanted. Asdescribed above, the resulting dynamic stabilization assembly will havereduced stiffness and greater range of motion than an embodiment inwhich the angle between the vertical rod and deflectable post isinvariant.

Referring to FIG. 10E, hinged coupling 1000 may also be used to securevertical rod 1030 to a standard bone anchor 1050 having a fixed threadedpost 1052. Hinged coupling 1000 may be used with or without lockingplate 1020 depending upon if it is desired to have vertical rod 1030pivot relative to the bone anchor 1050 after installation. When used inconjunction with a standard bone anchor, hinged coupling functions as apolyaxial head in that it allows the direction and angle of vertical rod1030 to be adjusted relative to bone anchor 1050 during installationthereby facilitating implantation of a spinal implant assembly.

FIGS. 11A-11D show an alternative ball-joint 1100 for connecting adeflection rod to a vertical rod according to an embodiment of theinvention. Ball-joint 1100 is designed to be mounted to the threadedproximal end of a deflectable post of a deflection rod. Ball-joint 1100may however be used with any deflection rod (or bone anchor) having asuitable proximal mount, for example, deflection rod 300 of FIGS. 3A-3H.

FIG. 11A shows an exploded view of ball-joint 1100. Ball-joint 1100includes split spherical bearing 1110, locking washer 1120 and avertical rod 1130. Ball-joint 1100 is held in place by a nut 1102.Vertical rod 1130 is configured to receive split spherical bearing 1110.One end of vertical rod 1130 includes a disc 1132 having a pocket 1134.Pocket 1134 is shaped like a segment of a sphere and has a largerdiameter in the interior than at the surface of disc 1132. Splitspherical bearing 1110 has the same diameter as the largest diameter ofpocket 1134. However, split spherical bearing 1110 has a centralaperture 1112 and a gap 1114 which allows split spherical bearing 1110to be compressed and inserted into pocket 1134. When split sphericalbearing 1110 is correctly positioned within pocket 1134 it is allowed toexpand into position. Expansion of bearing 1110 secures it within pocket1134, when split spherical bearing 1110 is ready for mounting.

FIG. 11B shows split spherical bearing 1110 mounted within pocket 1134of vertical rod 1130 ready for installation on a deflection rod 1101.Split spherical bearing 1110 protrudes on either side of disc 1132 toprovide adequate spacing for the movement of disc 1132. Central aperture1112 may be circular (as shown) or may be polygonal. Where aperture 1112is polygonal (non-circular) it may engage a similarly-shaped polygonalpost in such a way as to prevent rotation of bearing 1110 relative todeflectable post 1104. This is advantageous in that such rotation maycause nut 1102 to be loosened. Moreover, deflectable post 1104 of thedeflection rods of the present invention can typically rotate relativeto the bone anchor and thus rotation of spherical bearing 1110 is aredundant and therefore unnecessary degree of freedom.

As shown in FIG. 11B, deflection rod 1101 has at the end of deflectablepost 1104 a mount 1105 for receiving mounting plate 1116 of ball-joint1100. Mount 1105 has a lip 1106, a cylindrical portion 1107 and athreaded portion 1108. Lip 1106 provides a mechanical stop to catchbearing 1110. Cylindrical portion 1107 fits snuggly in aperture 1112 topreclude compression of split spherical bearing 1110 and therebypreventing split spherical bearing 1110 from being removed from pocket1134. In use, aperture 1112 is placed over mount 1105 until splitspherical bearing 1110 contacts lip 1106 and cylindrical portion 1107 isreceived with aperture 1112.

After bearing 1110 is positioned, vertical rod 1130 is oriented in thedesired direction and angle relative to deflectable post 1104.Typically, vertical rod 1130 is oriented by securing the free end 1131to another device on another vertebra of the spine, e.g. a bone screw,polyaxial screw, or deflection rod. When vertical rod 1130 is correctlypositioned, locking washer 1120 may be installed. Locking washer 1120has an aperture 1124 adapted to be received over mount 1105 ofdeflectable post 1104. Aperture 1124 is smaller than shoulder 1109 ofmount 1105. Locking washer 1120 has a lower lip 1122 designed, in oneorientation, to push down on bearing 1110. In the other orientation,locking washer 1120 is blocked by shoulder 1109 before locking washer1120 can compress bearing 1110. Bearing 1110 and locking washer 1120 aresecured in place by nut 1102 which engages threaded portion 1108 ofmount 1105 to secure vertical rod 1130 to deflectable post 1104.

FIG. 11C shows vertical rod 1130 secured to deflectable post 1104 byball-joint 1100 with vertical rod 1130 at a fixed angle relative todeflectable post 1104. As shown in FIG. 11C, split spherical bearing1110 is engaged on one side by lip 1106 of mount 1105. On the otherside, split spherical bearing 1110 is engaged by lip 1124 of lockingwasher 1120 which extends past shoulder 1109 of mount 1105. As nut 1102is tightened, split spherical bearing 1110 is compressed between washer1120 and lip 1106. Split spherical bearing 1110 is designed so that, inresponse to compression by nut 1102, it shrinks in height and expands indiameter. For example, split spherical bearing 1110 may be provided withinterior relief. When split spherical ring 1110 increases in diameter,it engages the surface of pocket 1134 sufficiently to preclude furthermovement of bearing 1110 relative to rod 1130. Thus, by tightening nut1102, vertical rod 1130 is secured to deflectable post 1104 and theangle between vertical rod 1130 and deflectable post 1104 is fixed.

In an alternative mode of installation shown in FIG. 11D, locking washer1120 may be omitted. In such case, nut 1102 is used to secure splitspherical bearing 1110 to mount 1105 of deflectable post 1104. In theabsence of locking washer 1120, nut 1102 cannot compress split sphericalbearing 1110 because nut 1102 does not extend beyond shoulder 1109 ofmount 1105. As a consequence, split spherical bearing 1110 is notcompressed and may therefore still rotate within pocket 1134 of verticalrod 1130. Thus, in this mode, vertical rod 1130 is free to pivot aboutbearing 1110 even after installation and securing of vertical rod 1130to deflectable post 1104 (see arrow 1138). Thus vertical rod 1130 isprovided with additional freedom of motion as finally implanted. Asdescribed above, the resulting dynamic stabilization assembly will havereduced stiffness and greater range of motion than an embodiment inwhich the angle between the vertical rod and deflectable post isinvariant. Ball joint 1100 may similarly be used (in either mode) tosecure vertical rod 1130 to a conventional bone anchor having a fixedthreaded post (see, e.g., bone anchor 1050 of FIG. 10E).

In alternative embodiments, shown in FIGS. 11E and 11F, a socket for asplit spherical bearing is mounted to the deflection rod itself. Avertical rod may be slipped into the split spherical bearing and may belocked in position and angle by two set screws. FIG. 11E shows anexploded view of the components which include socket 1150, bearing 1160and screws 1170 and 1172. Socket 1150 includes a lower fitting 1152(e.g. a threaded aperture) for attaching socket 1150 to deflection rod1101. Socket 1150 may also be bonded to deflection rod 1101 by e.g.laser welding, or may, in some cases, be formed in one piece with thedeflectable post of the deflection rod 1101. Socket 1150 has an internalpocket 1154 which is the same diameter as spherical bearing 1160. Socket1150 also has an upper threaded aperture 1156 for receiving set screw1170.

Spherical bearing 1160 has a spherical section 1162, a sleeve 1164, acentral passage 1166 and a threaded aperture 1168. Spherical section1162 is provided with a split or other structural relief mechanism thatallows it to be pressed into the pocket 1154 of socket 1150 (in theabsence of a vertical rod). Sleeve 1164 extends from one side ofspherical section 1162. Central passage 1166 extends through sleeve 1164and spherical section 1162 and is sized so that a vertical rod may slidetherethrough. Threaded aperture 1168 intersects passage 1166 such that,when inserted, locking set screw 1172 may secure a vertical rod 1180within passage 1166.

Spherical bearing 1160 is pressed into socket 1150 and socket 1150 issecured to deflection rod 1101 prior to implantation in a patient asshown in FIG. 11F. A vertical rod 1180 may then be inserted throughcentral passage 1166 of spherical bearing 1160 (shown in FIG. 11E). Whenvertical rod 1180 is at the desired position, set screw 1172 may betightened to secure the vertical rod 1180 within passage 1166. At thispoint, the vertical rod 1180 is secured to deflection rod 1101.Spherical bearing 1160 is secure within pocket 1154 of socket 1150because, with vertical rod in position, spherical section 1162 may nolonger be compressed sufficiently to remove it from pocket 1154.However, spherical section 1162 may still rotate within pocket 1154 andthus the angle between deflection rod 1101 and vertical rod 1180 canchange.

In some embodiments vertical rod 1180 may be oriented to the desiredangle and then screw 1170 may be tightened. Screw 1170 engages thespherical section 1162 of bearing 1160 and pushes it against the wall ofsocket 1154 thereby locking bearing 1160 in a fixed position. Aspreviously described, locking the angle of the vertical rod 1180relative to the deflection rod 1101 increases the stiffness of thesystem. However, if less stiffness and more range of motion is required,screw 1170 can be removed and bearing 1160 left free to rotate withinpocket 1154 after final assembly.

FIGS. 12A-12D show another ball-joint mechanism for connecting adeflection rod to a vertical rod according to an embodiment of theinvention. In the embodiment of FIGS. 12A-12D, the vertical rod isprovided with a locking ball receiver 1200 which can be secured to adeflection rod 1201 having a deflectable post 1204 which terminates in aball 1206. The locking ball receiver 1200 can be secured to the ball1206 in two modes. In one mode, the locking ball receiver 1200 issecured to the ball 1206 so that it cannot be removed from the ball 1206but can still rotate and pivot relative to the ball 1206. In the othermode, the locking ball receiver 1200 is secured to the ball 1206 suchthat it cannot be removed from the ball 1206 and nor can it rotate andpivot relative to the ball 1206.

FIG. 12A shows an exploded view of receiver 1200. Receiver 1200 includesclamp 1210, washer 1220 and is attached to vertical rod 1230. Thereceiver 1200 is positioned at one end of vertical rod 1230 and includesa pocket 1234. Pocket 1234 is shaped like a portion of a sphere havingthe same diameter as ball 1206. The entrance to pocket 1234 is the samediameter as ball 1206 or larger so that ball 1206 may be inserted intopocket 1234 during connection of vertical rod 1230 to deflectable post1204. Clamp 1210 has a pocket 1216 also shaped like a portion of asphere having the same diameter as ball 1206. Pocket 1216 can be movedaway from pocket 1234 to allow the insertion of ball 1206. Pocket 1216can be moved towards pocket 1234 to secure ball 1206 within pocket 1234.

Clamp 1210 is held in place by a screw 1202. During assembly, screw 1202is passed through an aperture 1212 in clamp 1210. Aperture 1212 islarger than screw 1202. Clamp 1210 fits within slot 1236 in receiver1200. Slot 1236 includes a ramp 1238 which engages a ramp 1214 of clamp1210. Engagement of ramp 1238 with ramp 1214 causes tightening of clamp1210 as clamp 1210 is brought closer to the bottom of slot 1236. Screw1202 passes into a threaded aperture 1235 in vertical rod 1230 so thattightening of screw 1202 draws clamp 1210 towards the bottom of slot1236. In some modes, washer 1220 is positioned between clamp 1210 andthe bottom of slot 1236 thereby spacing clamp 1210 from the bottom ofslot 1236 and limiting the clamping action of clamp 1210.

As shown in FIG. 12B, deflectable post 1204 has at the end ofdeflectable post 1204 ball 1206 to which receiver 1200 may be mounted.Ball 1206 may be formed in one piece with deflectable post 1204 or maybe formed separately and subsequently securely attached. FIG. 12B showsreceiver 1200 positioned over ball 1206 of a deflectable post 1204. Ball1206 is slipped into pocket 1234 with clamp 1210 removed or looselyattached to vertical rod 1230. After ball 1206 is positioned withinpocket 1234, vertical rod 1230 is oriented in the desired direction andangle relative to deflectable post 1204. Typically vertical rod 1230 isoriented by securing the free end 1231 to another device on anothervertebra of the spine, e.g. a bone screw, polyaxial screw, or deflectionrod. When vertical rod 1230 is correctly positioned, screw 1202 may betightened. As screw 1202 is tightened, ramp 1238 pushes on ramp 1214 topush pocket 1216 towards pocket 1234. If washer 1220 is present, theapproach of pocket 1216 towards pocket 1234 is limited by the washer1220 so that clamp 1210 does not lock to ball 1206. If washer 1220 isabsent, clamp 1210 is forced into contact with ball 1206 by thetightening of screw 1202.

FIG. 12C shows a sectional view of vertical rod 1230 secured todeflectable post 1204 by receiver 1200. As shown in FIG. 12C, ball 1206is trapped between pocket 1216 of clamp 1210 and pocket 1234 of verticalrod 1230. Pockets 1216 and 1234 combine to form a pocket which trapsball 1206. As screw 1202 is tightened, pocket 1216 is pushed furthertowards pocket 1234 by the interaction or ramps 1214 and 1238. However,where washer 1220 is present, the approach of pocket 1216 towards pocket1234 is halted before any clamping pressure is applied to ball 1206.Thus, vertical rod 1230 may still rotate and pivot relative to ball 1206after screw 1202 has been tightened. Thus, vertical rod 1230 is providedwith an additional degree of freedom of motion as finally implanted. Asdescribed above, the resulting dynamic stabilization assembly will havereduced stiffness and greater range of motion than an embodiment inwhich the angle between the vertical rod and deflectable post isinvariant.

In an alternative mode of installation shown in FIG. 12D, washer 1220(not shown) may be omitted. In the absence of washer 1220, clamp 1210can be moved closer to the bottom of slot 1236. As screw 1202 istightened, pocket 1216 is pushed further towards pocket 1234 by theinteraction or ramps 1214 and 1238. Now that washer 1220 (not shown) isabsent, the approach of pocket 1216 towards pocket 1234 is not halteduntil clamping pressure is applied to ball 1206. As a consequence,vertical rod 1230 is fixed to ball 1206 and cannot rotate or pivotrelative to ball 1206. As described above, the resulting dynamicstabilization assembly will have increased stiffness but less range ofmotion than an embodiment in which the angle between the vertical rodand deflectable post may vary. The receiver may similarly be used (ineither mode) to secure vertical rod 1230 to a standard bone anchorhaving a fixed threaded post e.g. bone anchor 1050 of FIG. 10E).

FIGS. 13A and 13B show another mechanism for connecting a deflection rodto a vertical rod according to an embodiment of the invention. FIGS. 13Aand 13B show a deflection rod having a pivoting head according to anembodiment of the invention. In the embodiment of FIGS. 13A-13B, thedeflection rod has an integrated connector 1300 which can be secured toa vertical rod. The connector 1300 can be secured to the vertical rod intwo modes. In one mode the connector 1300 is secured to the vertical rodso that it cannot be removed from the connector 1300 but can still pivotrelative to the deflection rod. In the other mode, the connector 1300 issecured to the vertical rod such that it cannot be removed from theconnector 1300 and nor can it pivot relative to the deflection rod.

FIG. 13A shows an exploded view of connector 1300. Connector 1300includes saddle 1310, plunger 1320, set screw 1330, pivot pin 1332 andis attached to a deflectable post 1340 of a deflection rod 1302.Deflectable post 1340 has a disk-shaped mount 1342 at the proximal end.Mount 1342 has a central aperture 1344 sized to receive pivot pin 1332.Saddle 1310 is approximately tube shaped with a bore 1316 which passesthrough the long axis of saddle 1310. At one end of saddle 1310 is aclevis 1312 sized to receive disk-shaped mount 1342. Clevis 1312 has anaperture 1314 which passes through clevis 1312 and is sized to receivepivot pin 1332. To assemble connector 1300, disk-shaped mount 1342 isinserted into clevis 1312 and pivot pin 1332 is passed through aperture1314 and aperture 1344. Pivot pin 1332 is then secured to one or bothsides of clevis 1312 using mechanical means and/or bonding e.g. laserwelding. Saddle 1310 is then free to pivot relative to deflectable post1340 around the axis of pivot pin 1332.

Bore 1316 now communicates with disk-shaped mount 1342. A plunger 1320may now be introduced into bore 1316. Plunger 1320 has surface features,for example, ribs 1326 designed to engage the surface features ofdisk-shaped mount 1342. The end of saddle 1310 opposite clevis 1312 hasa slot 1318 which passes therethrough. Slot 1318 is sized to receive avertical rod. At the closed end slot 1318 intersect the position ofplunger 1320. At the open end of slot 1318, bore 1316 is threaded toreceive set screw 1330. When a vertical rod (not shown) is inserted intoslot 1318, tightening of set screw 1330 forces the vertical rod down inslot 1318 towards plunger 1320 which is in turn pushed down into contactwith disk-shaped mount 1342.

FIG. 13B shows connector 1300 assembled with a deflection rod 1302 andbone anchor 1304. As assembled, connector 1300 may pivot about the axisof pivot pin 1332 as shown by arrow 1306. Connector 1300 may also rotatearound the long axis of bone anchor 1304 as shown by arrow 1308.Rotation 1308 is possible because deflectable post 1340 may rotatearound its long axis within deflection rod 1302.

FIGS. 14A and 14B show another mechanism for connecting a deflection rodto a vertical rod according to an embodiment of the invention. FIGS. 14Aand 14B show a deflection rod having a pivoting head according to anembodiment of the invention. In the embodiment of FIGS. 14A and 14B, thedeflection rod has an integrated connector 1400 which can be secured toa vertical rod. The connector 1400 can be secured to the vertical rod intwo modes. In one mode, the connector 1400 is secured to the verticalrod so that it cannot be removed from the connector 1400 but can stillpivot relative to the deflection rod. In the other mode, the connector1400 is secured to the vertical rod such that it cannot be removed fromthe connector 1400 and nor can it pivot relative to the deflection rod.

FIG. 14A shows an exploded view of connector 1400. Connector 1400includes saddle 1410, plunger 1420, set screw 1430, pivot pins 1432,1434 and is attached to a deflectable post 1440 of a deflection rod 1402(see FIG. 14B). Deflectable post 1440 has clevis 1442 at the proximalend. Clevis 1442 has an aperture 1444 which passes through both arms ofclevis 1442 and is configured to receive pivot pins 1432, 1434. Saddle1410 is approximately tube shaped with a bore 1416 which passes throughthe long axis of saddle 1410. Clevis 1442 has an aperture 1414 whichpasses through clevis 1412 and is sized to receive pivot pins 1432,1434. To assemble connector 1400, saddle 1410 is inserted into clevis1442 and pivot pins 1432, 1434 are passed through apertures 1444 intoapertures 1414. Pivot pins 1432, 1434 is then secured to both sides ofclevis 1442 using mechanical means and/or bonding e.g. laser welding.Saddle 1410 is then free to pivot relative to deflectable post 1440around the axis of pivot pins 1432, 1434.

Bore 1416 now communicates with ribbed surface 1446 of clevis 1442. Aplunger 1420 may now be introduced into bore 1416. Plunger 1420 hassurface features, for example, ribs 1426 designed to engage the ribbedsurface 1446 of clevis 1442. The proximal end of saddle 1410 has a slot1418 which passes therethrough. Slot 1418 is sized to receive a verticalrod (not shown). At the closed end, slot 1418 intersects the position ofplunger 1420. At the open end of slot 1418, bore 1416 is threaded toreceive set screw 1430. When a vertical rod (not shown) is inserted intoslot 1418, tightening of set screw 1430 forces the vertical rod down inslot 1418 towards plunger 1420 which is in turn pushed down into contactwith ribbed surface 1446 of clevis 1442. Contact between plunger 1420and clevis 1442 locks saddle 1410 so that it can no longer pivotrelative to the plunger. However, in an alternative configuration,plunger 1420 is omitted, and set screw 1430 can be used to lock thevertical rod (not shown) to the saddle 1410 while still allowing saddle1410 to pivot with respect to clevis 1442.

FIG. 14B shows connector 1400 assembled with a deflection rod 1402 andbone anchor 1404. As assembled, connector 1400 may pivot about the axisof pivot pin 1432 as shown by arrow 1406. Connector 1400 may also rotatearound the long axis of bone anchor 1404 as shown by arrow 1408.Rotation 1408 is possible because deflectable post 1440 may rotatearound its long axis within deflection rod 1402. A vertical rod (notshown) may be inserted into slot 1418. Tightening set screw 1430 securesthe vertical rod to the saddle 1410. If plunger 1420 is present,tightening set screw 1430 also locks the relative positions of saddle1410 and clevis 1442 preventing pivoting after implantation. If plunger1420 is absent, tightening set screw 1430 does not lock saddle 1410 toclevis 1442 and the saddle 1410 may still pivot after implantation. Asdescribed above, the resulting dynamic stabilization assembly will havereduced stiffness and greater range of motion in an embodiment thatallows pivoting between the vertical rod and deflectable post rod afterimplantation than an embodiment in which the angle between the verticalrod and deflectable post is locked.

FIG. 15A illustrates a preferred embodiment of the deflection rod 300and bone anchor 320 of FIG. 3A. As shown in FIG. 15A, mount 314 ofdeflection rod 300 includes a polygonal section 1502 for secure mountingto a vertical rod component. Polygonal section 1502 may be hexagonal,octagonal or the like. Polygonal section 1502 is shaped to match theshape of a receiver in the vertical rod component (not shown) such thatwhen the two are mounted together there will be no rotation. As shown inFIG. 15A, the proximal end of mount 314 is threaded 1504 to receive afastener to secure a vertical rod component to mount 314. Deflection rod300 is otherwise as previously described.

FIG. 15A also shows a preferred embodiment of vertical rod 1510 for usewith deflection rod 300. As shown in FIG. 15A, vertical rod 1510comprises a rod 1511 which is preferably a 5.5 mm diameter titanium rod.Vertical rod 1510 has a pocket 1512 at one end sized to receive a ball1520. Ball 1520 is preferably a cobalt chrome ball. Ball 1520 has apolygonal aperture 1522 designed to closely engage the polygonal section1502 of mount 314. Ball 1520 is inserted into pocket 1512 and securedinto place with threaded cap 1530. Pocket 1512 is threaded to receivecap 1530. Ball 1520 is placed in pocket 1512 and then cap 1530 isscrewed into the threaded portion of pocket 1512. Cap 1530 is preferablytitanium and may be laser welded or otherwise secured to vertical rod1510 after assembly. The components of vertical rod 1510—titanium rod1511, titanium cap 1530 and cobalt chrome ball 1520 are assembled priorto use.

FIGS. 15B and 15C shows a sectional view through vertical rod 1510 afterassembly. FIG. 15B shows ball 1520 positioned within pocket 1512 of rod1511. As shown in FIG. 15B cap 1530 and pocket 1512 capture ball 1530such that it cannot be removed from vertical rod 1510. Ball 1530 can,however, rotate 360 degrees around the axis of aperture 1522 as shown byarrow 1550. This allows vertical rod 1510 to rotate 360 degrees aroundthe long axis of the deflection rod or bone anchor to which ball 1530 ismounted. Ball 1530 can also tilt from the position shown in FIG. 15B asshown in FIG. 15C by arrows 1552. In a preferred embodiment ball 1530can tilt 15 degrees in any direction therefore allowing vertical rod1510 to tilt 15 degrees from perpendicular relative to the deflectionrod or bone anchor to which ball 1530 is mounted. Note that the mount314 and a nut to secure the vertical rod 1510 to mount 314 are designedso not as to interfere with the range of motion either in rotation ortilting.

Vertical rod 1510 may be used with a standard bone anchor, a deflectionrod and bone anchor (for example bone anchor 320 and deflection rod 300of FIG. 15A), or a polyaxial screw. Likewise, the assembly of deflectionrod 300 and bone anchor 320 of FIG. 15A may be utilized with verticalrod 1510, but may also be utilized in conjunction with a vertical rodnot having a ball joint.

FIGS. 16A and 16B show an alternative embodiment of deflection rod 1600which includes mount 1670 for connecting the deflection rod to avertical rod. As shown in FIG. 16A, mount 1670 includes a circular plate1674; the face of which is parallel to the longitudinal axis ofdeflectable post 1604. A threaded pin 1672 projects from the center ofcircular plate 1674. Threaded pin 1672 is perpendicular to thelongitudinal axis of deflectable post 1604. On the face of circularplate 1674 surrounding pin 1672 are a plurality of radial splines 1676.

Mount 1670 is designed to mate with vertical rod 1680 as also shown inFIG. 16A. Vertical rod 1680 has at one end a circular plate 1684; theface of which is parallel to the longitudinal axis of vertical rod 1680.An aperture 1682 passes through the center of circular plate 1684 and issized to receive threaded pin 1672. Aperture 1682 is perpendicular tothe longitudinal axis of vertical rod 1680. On the face of circularplate 1684 surrounding aperture 1682 are a plurality of radial splines1686. The radial splines 1686 of vertical rod 1680 are designed to matewith and engage the splines 1676 of mount 1670.

As shown in FIG. 16B, aperture 1682 of vertical rod 1680 is receivedover threaded pin 1672 of mount 1670. The angle of vertical rod 1680 isrelative to deflectable post 1604 may be adjusted as shown by arrow1692. Adjustment of the relative angle of deflectable post 1604 andvertical rod 1680 combined with the ability of deflectable post 1604 torotate about its long axis (as shown by arrow 1694) is relative to boneanchor 1620 provides two degrees of freedom and thus sufficientflexibility of installation to align vertical rod 1680 with a boneanchor implanted in another vertebrae. As shown in FIG. 16B, a nut 1690engages threaded pin 1672 to secure plate 1674 to plate 1684. Splines1676 of plate 1674 are arranged facing splines 1686 of plate 1684. Whennut 1690 is tightened, splines 1686 engage splines 1676 to preventrotation of vertical rod 1680 about pin 1672. Thus, when nut 1690 istightened, the angle between deflectable post 1604 and vertical rod 1680is fixed. The vertical rod mounting mechanism of FIGS. 16A and 16B maybe readily applied to any of the deflection rod systems describedherein.

Further Alternative Deflection Rods/Loading Rods

FIG. 17A shows an alternative deflection rod 1700. FIG. 17A shows thedeflectable post 304 and also shows (in dotted lines) the position ofdeflectable post 304 upon deflection. Deflection rod 1700 has most ofthe same components as deflection rod 300 of FIGS. 3A-3E. Applicantsfound, that upon deflection of deflectable post 304 of deflection rod300 of FIGS. 3A-3E there was a propensity for sleeve 300 of FIGS. 3A-3Eto expand longitudinally during compression by deflectable post 304 andbecome trapped between deflection rod 300 and collar 310. Deflection rod1700 therefore has a modified sleeve 1706 and modified collar 1710.

As shown in FIG. 17A, sleeve 1706 is provided with a relief 1720 on theupper surface. Relief 1720 allows space for longitudinal expansion ofsleeve 1706 during radial compression of sleeve 1706 by post 304. Thus,sleeve 1706 does not become trapped between deflectable post 304 andcontact surface 1713 of collar 1710 upon deflection of deflectable post304 (as shown by dotted lines). This design reduces wear on sleeve 1706and ensures that deflectable post 304 may freely travel through itsdesigned range of deflection.

As shown in FIG. 17A, collar 1710 may also be provided with a relief1712 to further assist in preventing sleeve 1706 from becoming trappedbetween collar 1710 and deflectable post 304. Additionally, sleeve 1706may be provided with a lower relief 1722 in order to prevent sleeve 1706from being trapped between deflectable post 304 and shield 308 in theregion of retainer 302.

FIG. 17B shows a perspective view of sleeve 1706. Sleeve 1706 is made ofa compliant material which permits movement of deflectable post 304relative to shield 308 (FIG. 17A). The sleeve 1706 effectively controlsthe deflection of the deflectable post 304. Sleeve 1706 is preferablymade of a compliant biocompatible polymer such as PCU by way of exampleonly. The properties of the material and dimensions of sleeve 1706 areselected to achieve the desired force/deflection characteristics fordeflectable post 304 (FIG. 17A). In a preferred embodiment, the sleeveis made of PCU, is 2 mm thick when uncompressed and may be compressed toabout 1 mm in thickness by deflection of the post.

As can be seen from FIG. 17B, relief 1720 forms a conical depression inthe proximal surface of sleeve 1706 surrounding the central aperture1707 which receives deflectable post 304 (not shown). The removal ofmaterial from the proximal surface of sleeve 1706 (as compared withsleeve 306 of FIGS. 3A-3E) allows room for expansion of sleeve 1706without sleeve 1706 becoming trapped between deflectable post 304 andcollar 1710 (FIG. 17A). Sleeve 1706 may also be shaped to modify thecompliance of sleeve 1706, for example by providing additional regionsof relief or voids within the body of sleeve 1706 (see, e.g. flutes 307of FIG. 3A).

FIG. 17C shows a perspective view of an alternative sleeve 1706 c, alsohaving a relief 1720 c in the proximal surface surrounding the centralaperture 1707 c which receives deflectable post 304. The relief 1720 cis curved—the curve extending from the perimeter of central aperture1707 c to the flat region 1705 c of sleeve 1706 c which is engaged bycollar 1710 upon assembly. In this embodiment, the outer circumferenceof sleeve 1706 c is provided with a plurality of scallops 1704 c. Thescallops are larger in depth at the proximal end of sleeve 1706 c (topin FIG. 17C) and taper towards this distal end of sleeve 1706 c (bottomin FIG. 17C). Scallops 1704 c serve to make the sleeve 1706 c morecompliant/flexible. In the sleeve 1706 c, the scallops make the proximalend of sleeve 1706 c more compliant than the distal end of sleeve 1706c. This is advantageous as the geometry of deflection rod 1700 resultsin greater compression at the proximal end of sleeve 1706 c than at thedistal end of sleeve 1706 c. Increasing the flexibility of the proximalend of sleeve 1706 c thus serves to balance out the forces applied tothe deflectable post by the proximal and distal regions of sleeve 1706 callowing for a more even distribution of loading and “work” within thesleeve 1706 c. Scallops 1704 c also serve to reduce the volume ofmaterial at the proximal end of sleeve 1706 c. During deflection ofdeflectable post 304 (FIG. 17A) the sleeve 1706 c can expand into thevoid left by scallops 1704 c further reducing the possibility thatsleeve 1706 c will become trapped between deflectable post 304 andcollar 1710.

FIG. 17D shows a perspective view of another alternative sleeve 1706 d.Sleeve 1706 d has a relief 1720 d in the proximal surface surroundingthe central aperture 1707 d. Relief 1720 d takes the form of a conicaldepression in the proximal surface of sleeve 1706 d. Sleeve 1706 d alsohas a plurality of voids 1704 d which penetrate from the proximalsurface of sleeve 1706 d into the body of sleeve 1706 d along an axisparallel to the axis of central aperture 1707 d. As shown in FIG. 17D,voids 1704 d are circular in section. Voids 1704 d may be, for examplecylindrical apertures which pass all the way through sleeve 1706 d.Alternatively, the voids 1704 d may be cylindrical apertures which passpart of the way but not all of the way through sleeve 1706 d.Alternatively, voids 1704 d may be conical voids in which the size ofthe void diminishes as the void passes through sleeve 1706 d. The voidsserve similar functions as scallops 1704 c of FIG. 17C. For example,voids 1704 d serve to increase the compliance of the material/region ofsleeve 1706 d and provide space for the sleeve to be pushed into bydeflectable post 304 thereby avoiding pinching between deflectable post304 and collar 1710 (See FIG. 17A).

FIG. 17E shows a sectional view of another alternative sleeve 1706 e. Asshown in FIG. 17E, sleeve 1706 e includes a plurality of voids 1704 ewithin the body of sleeve 1706 e. Voids 1706 e spiral out from aposition adjacent central aperture 1707 e towards the outer edge ofsleeve 1706 e. As shown, voids 1704 e may be larger towards the outeredge of sleeve 1706 e where there is more material. As previouslydiscussed voids 1704 e may have a different cross-section at differentlevels in sleeve 1706 e. For example, voids 1704 e may have a largerarea at the proximal end of sleeve 1706 e (closest to collar 1710 ofFIG. 17A) than at the distal end of sleeve (closest to retainer 302 ofFIG. 17A) thereby increasing the flexibility of sleeve 1706 e wheredeflectable post 304 has the greatest amount of deflection. The voids1704 e serve similar functions as scallops 1704 c of FIG. 17C. Forexample, the voids 1704 e serve to increase the compliance of thematerial/region of sleeve 1706 e and provide space for the sleeve 1706 eto be pushed into by deflectable post 304 thereby avoiding pinchingbetween deflectable post 304 and collar 1710 (See FIG. 17A).

The sleeves 1706, 1706 c, 1706 d and 1706 e show alternativeconfigurations designed to achieve the function of controlling themovement of a deflectable post. Such sleeves may be incorporated intoany of the deflection rod systems described herein. Different designsand combinations of relief and voids than those illustrated may beutilized to adjust the flexibility of the sleeve and prevent pinching ofthe sleeve between the deflectable post and other components of thedeflection rod system.

FIGS. 18A-18D illustrate another alternative deflection rod 1800. FIG.18A shows an exploded view of alternative deflection rod 1800. FIG. 18Bshows the deflection rod assembled with a bone anchor. FIGS. 18C-18Dshow sectional views of deflection rod 1800 and illustrate deflection ofthe deflection rod. As shown in FIG. 18A, deflection rod 1800 includesfour components: ball-shaped retainer 1802, deflectable post 1804,o-ring 1806, cap 1810.

Deflectable post 1804 has a retainer 1802 at one end. Retainer 1802 is aspherical structure formed in one piece with deflectable post 1804. Atthe other end of deflectable post 1804 is a mount 1814. Mount 1814, inthis embodiment, is suitable for connecting to a vertical rod. A ballmay be used in place of mount 1814 as previously described. In thisembodiment, mount 1814 is also formed in one piece with deflectable post1804 and retainer 1802. In alternative embodiments, deflectable post1804 may be formed separately from and securely attached to one or moreof mount 1814 and retainer 1802 by laser welding, soldering or otherbonding technology. Alternatively, deflectable post 1804 may be formedseparately and mechanically engage one or more of mount 1814 andretainer 1802 using, for example, threads. For example, a lock ring,toothed locking washer, cotter pin or other mechanical device can beused to secure deflectable post 1804 to one or more of mount 1814 andretainer 1802.

As shown in FIG. 18A, mount 1814 may be provided with a hexagonalextension 1815 which may be utilized when securing a vertical rod tomount 1814. Extension 1815 may be gripped by a wrench to preventrotation of mount 1814 as a nut is tightened onto the threaded region ofmount 1814. Extension 1815 may be formed in one piece with mount 1814. Agroove 1816 between mount 1814 and extension 1815 reduces thecross-section of material such that extension 1815 breaks away frommount 1814 when a desired torque is achieved. In this way a vertical rodmay be secured to mount 1814 and then extension 1815 removed.

Deflection rod 1800 is configured to be mounted in a bone anchor 1820,which comprises a bone screw 1822 connected to a housing 1830. Housing1830 has a cavity 1832 oriented along the axis of bone anchor 1820 atthe proximal end and configured to receive deflection rod 1800. Housing1830 also has an outer surface 1834 adapted for mounting a componente.g. an offset connector. Housing 1830 may, in some embodiments, becylindrical as previously described. As shown in FIG. 18A, outer surface1834 of housing 1830 is provided with splines/flutes 1836.Splines/flutes 1836 may be engaged by a driver that mates withsplines/flutes 1836 for implanting bone anchor 1820.

Cap 1810, in this embodiment is designed to perform multiple functionsincluding securing o-ring 1806 as well as securing retainer 1802 incavity 1832 of bone anchor 1820. Cap 1810, by integrating the functionsof the collar and sleeve, reduces the complexity of the deflection rod1800 and also increases the strength of the deflection rod 1800 orallows a reduction in size for the same strength. Cap 1810 comprises acylindrical shield section 1808 connected to a collar section 1809. Cap1810 is designed to mate with aperture 1832 of housing 1830. The shieldsection 1808 and collar section 1809 are preferably formed in one piece,however, they may be formed separately and then secured together. Shieldsection 1808 is threaded adjacent collar section 1809 in order to engagethreaded aperture 1832. Cap 1810 may alternatively or additionally bejoined to housing 1830 by for example laser welding.

O-ring 1806 is made of a compliant material. O-ring 1806 fits within agroove 1805 of shield 1808 of cap 1810 (see FIG. 18C). O-ring 1806 iscircular in section but may also be differently shaped to modify thecharacteristics of o-ring 1806, including, for example, compliance (seeFIGS. 8E-8G). O-ring 1806 has a central aperture 1807 through whichdeflectable post 1804 may be positioned. O-ring 1806 permits movement ofdeflectable post 1804 relative to shield 1808. The o-ring 1806effectively controls and limits the deflection of the deflectable post1804. O-ring 1806 is preferably made of a compliant biocompatiblepolymer such as PCU by way of example only. The properties of thematerial and dimensions of the o-ring 1806 are selected to achieve thedesired force/deflection characteristics for deflectable post 1804.O-ring 1806 may be made of a compliant implantable polymer having thedesired compliance and durability. For example, o-ring 1806 may be madefrom polycarbonate urethane. In a preferred embodiment, o-ring 1806 maybe made from BIONATE®. If the o-ring 1806 is comprised of Bionate®, apolycarbonate urethane or other hydrophilic polymer, the o-ring 1806 canact as a fluid lubricated bearing.

Referring now to FIG. 18B, which shows a perspective view of adeflection rod 1800 assembled with a bone anchor 1820. When assembled,deflectable post 1804 is positioned within cap 1810 which is positionedwithin housing 1830 of bone anchor 1820. O-ring 1806 (not seen in thisview) is first positioned within shield 1808 of cap 1810. Deflectablepost 1804 is then positioned through o-ring 1806 and cap 1810.Deflectable post 1804, o-ring 1806 and cap 1810 are then positionedwithin the cavity 1832 of housing 1830. The cap 1810 is then secured tothe threaded proximal end of cavity 1832. Cap 1810 has two sockets 1811for receiving the pins of a pin wrench to allow cap 1810 to be tightenedto housing 1830. Cap 1810 may be, alternatively or additionally, laserwelded to housing 1830 after installation to secure the components. Cap1810 secures deflectable post 1804 and o-ring 1806 within cavity 1832 ofbone anchor 1820. (See FIG. 18C).

As shown in FIG. 18B, deflectable post 1804 extends out of housing 1830and cap 1810 such that mount 1814 is accessible for connection to avertical rod. There is a gap between deflectable post 1804 and cap 1810which permits deflection of deflectable post 1804 through a predefinedrange before deflection is limited by contact with cap 1810.

FIG. 18C shows a sectional view of a deflection rod 1800 assembled witha bone anchor 1820 along the axis indicated by line C-C of FIG. 18B.Retainer 1802 fits into a hemispherical pocket 1839 in the bottom ofcavity 1832 of housing 1830. The bottom edge of cap 1810 includes aflange 1815 which secures ball-shaped retainer 1802 within hemisphericalpocket 1839 while allowing rotation of ball-shaped retainer 1802. Collar1809 thereby secures both retainer 1802 and o-ring 1806 within housing1830. If the o-ring 1806 is comprised of Bionate®, a polycarbonateurethane or other hydrophilic polymer, the o-ring 1806 can act as afluid lubricated bearing and allow the deflectable post 1804 to alsorotate about the longitudinal axis of the deflectable post 1804 and thebone anchor 1820. Other materials and configurations can also allow thepost to rotate about the longitudinal axis of the post and the boneanchor.

As shown in FIG. 18C, o-ring 1806 occupies the space between deflectablepost 1804 and shield 1808 of cap 1810. O-ring 1806 is secured withingroove 1805 of cap 1810. O-ring 1806 may be compressed by deflection ofdeflectable post 1804 towards shield 1808 in any direction. O-ring 1806is circular in section and this configuration avoids pinching of o-ring1806 between deflectable post 1804 and shield 1808. The circular sectionor o-ring 1806 reduces the area of contact between deflectable post 1804and o-ring 1806 thereby reducing wear. O-ring 1806 may be slightlylarger than the space between the deflectable post and the bottom ofgroove 1805. This provides preload which reduces slack in the deflectioncharacteristics of the deflection rod 1800. The preload is also usefulto reduce the occurrence of slack if the o-ring becomes worn during use.

FIG. 18D illustrates the deflection of deflectable post 1804. Applying aforce to mount 1814 causes deflection of deflectable post 1804 ofdeflection rod 1800. Initially deflectable post 1804 pivots about apivot point 1803 indicated by an X. Deflectable post 1804 may pivotabout pivot point 1803 in any direction. Concurrently or alternatively,deflectable post 1804 can rotate about the long axis of deflectable post1804 (which also passes through pivot point 1803). In this embodiment,pivot point 1803 is located at the center of ball-shaped retainer 1802.As shown in FIG. 18D, deflection of deflectable post 1804 compresses thematerial of o-ring 1806. O-ring 1806 is compressed into groove 1805.Groove 1805 may be slightly wider than necessary to accommodate o-ring1806 in order that o-ring 1806 may expand axially while being compressedradially. The extra space in groove 1805 reduces the possibility thato-ring 1806 will become pinched between deflectable post 1804 and theinside of cap 1810. The force required to deflect deflectable post 1804depends upon the dimensions of deflectable post 1804, o-ring 1806,groove 1805 and shield 1808 of cap 1810 as well as the attributes of thematerial of o-ring 1806. The o-ring exerts a centering force back ondeflectable post 1804 pushing it back towards a position coaxial withbone anchor 1820.

After further deflection, deflectable post 1804 comes into contact withlimit surface 1813 of collar 1809. Limit surface 1813 is oriented suchthat when deflectable post 1804 makes contact with limit surface 1813,the contact is distributed over an area to reduce stress on deflectablepost 1804. After deflectable post 1804 comes into contact with limitsurface 1813, further deflection requires deformation (bending) ofdeflectable post 1804. In a preferred embodiment, deflectable post 1804is a titanium post 5 mm in diameter. Deflectable post 1804 is relativelystiff, and the force required to deflect deflectable post 1804 thereforeincreases significantly after contact of deflectable post 1804 with cap1810. In a preferred embodiment, deflectable post 1804 may deflect from0.5 mm to 2 mm in any direction before making contact with limit surface1813. More preferably, deflectable post 1804 may deflect approximately 1mm before making contact with limit surface 1813.

The inner diameter of the cap 1810 may be different in different caps sothat the distance between limit surface 1813 and deflectable post 1804is different in different deflection rods. This allows for themanufacture of deflection rods having a larger or smaller range ofdeflection before contact between the post 1804 and the limit surface1813. In this way deflection rods may be manufactured having differentranges of motion. Moreover the distance between limit surface 1813 anddeflectable post 1804 need not be the same in all directions such thatthe range of motion of the deflection rod is different in differentdirections.

Referring to FIG. 18D, as load or force is first applied to thedeflection rod 1800 by the spine, the deflection of deflectable post1804 responds about linearly to the increase in the load during thephase when deflection of deflectable post 1804 causes compression ofo-ring 1806. After about 1 mm of deflection, deflectable post 1804contacts limit surface 1813 and the deflection rod becomes substantiallystiffer. A greater amount of load or force needs to be placed on thedeflection rod in order to obtain the same amount of incrementaldeflection that was realized prior to this point because furtherdeflection requires bending of deflectable post 1804. The amount ofdeflection caused by the load applied is a non-linear function, in thisembodiment. The deflection rod provides a range of motion where the loadsupported increases about linearly as the deflection increases and thenwith increased deflection the load supported increases more rapidly(upon contact of the post with the limit surface). Alternatively, ifdesired, this embodiment could be designed such that the rate of changeof the amount of deflection could be a linear function for a largerrange of motion by; for example, increasing the distance between limitsurface 1813 and deflectable post 1804.

FIG. 18E shows a partial sectional view of o-ring 1806. As shown in FIG.18E, o-ring 1806 is circular in section. The circular section helpsreduce the area of contact with deflectable post 1804 (See (FIGS.18A-18B). The reduced contact results in less wear. However, the o-ringmay have different shapes. As shown in FIG. 18F, an o-ring 1856 may havea flat outer edge 1852 for engaging groove 1805 of cap 1810 while stillhaving a curved inner edge 1854 for engaging deflectable post 1804. Ano-ring 1856 of this design would be less compliant than o-ring 1806, allother factors being equal. Thus, o-ring 1806 would exert a greaterreturn force upon a deflectable post 1804 for the same amount ofdeflection.

As shown in FIG. 18G, the compliance of an o-ring 1866 may also bemodified by having a void 1862 (or voids) within o-ring 1866. The voidsmay contain gas or other fluid and thereby provide pneumatic orhydraulic force/deflection characteristics. As previously discussed,o-rings may be designed that exhibit anisotropic force deflectioncharacteristics by for example having anisotropic variations in shape ormaterial.

FIG. 18H shows another alternative o-ring 1876. O-ring 1876 has a flatouter edge 1872 for engaging groove 1805. O-ring 1876 has a curved inneredge 1874 for engaging deflectable post 1804. O-ring 1876 also hasangled reliefs 1877, 1878, one either side of inner edge 1874, whichserve to reduce the area of contact between o-ring 1876 and deflectablepost 1804.

FIGS. 19A-19D illustrate a preferred embodiment of a bone anchor fordynamic stabilization of the spine having an in-line deflection rodassembly 1900 built into a bone anchor 1920. FIGS. 19A-19D also show apreferred embodiment of a dynamic vertical rod 1950 for use with thedeflection rod assembly 1900. FIG. 19A shows an exploded view of dynamicvertical rod 1950 and deflection rod assembly 1900 built into a boneanchor 1920. FIG. 19B shows the deflection rod assembled with a boneanchor and dynamic vertical rod. FIGS. 19C-19D show sectional views ofdeflection rod assembly 1900 and illustrate deflection of the deflectionrod. FIGS. 19E and 19F show enlarged views of components of deflectionrod assembly 1900.

Referring now to FIG. 19A, deflection rod assembly 1900 includes, inthis embodiment, four components: ball-shaped retainer 1902, deflectablepost 1904, o-ring 1906 and cap 1910. Retainer 1902 and deflectable post1904 form the ball rod 1960 as shown in FIG. 19 b and will be discussedherein. Deflection rod assembly 1900 is configured to be mounted in abone anchor 1920, which comprises a bone screw 1922 connected to ahousing 1930. Housing 1930 has a cavity 1932 oriented along the axis ofbone anchor 1920 at the proximal end and configured to receivedeflection rod assembly 1900. In this embodiment, housing 1930 istruncated relative to the bone anchor 1820 of FIG. 18A. Housing 1930 andbone anchor 1920 are preferably formed in one piece from titanium ortitanium alloy.

Deflectable post 1904 has a retainer 1902 at one end. Retainer 1902 is aspherical structure formed in one piece with deflectable post 1904. Atthe other end of deflectable post 1904 is a mount 1914. As shown in FIG.19A, mount 1914 is a low profile mount configured to fit within a balljoint 1940 of a vertical rod component. Mount 1914 is configured to besecured to a spinal vertical rod component and comprises a threadedcylinder to which the vertical rod component may be secured. Anintegrated ball may be used in place of mount 1914 as previouslydescribed. Mount 1914 includes a male hex extension 1912 which may beengaged by a tool to hold mount 1914 during attachment to a verticalrod. At the proximal end of male hex extension is a feature for securinghex extension 1912 in a tool, in this embodiment a nipple 1918. In apreferred embodiment mount 1914, deflectable post 1904, retainer 1902,hex extension 1912 and nipple 1918 are made in one piece from cobaltchrome allowing for enhanced wear characteristics. Alternatively,titanium or titanium alloy may be used either alone or with a cobaltchrome coating. The combination of mount 1914, deflectable post 1904 andretainer 1902 may be referred to as a ball rod 1960.

FIG. 19G shows an enlarged perspective view of ball rod 1960. Referringto FIG. 19G, ball rod 1960 is formed in one piece with four mainsections, which are, starting from the distal end, ball-shaped retainer1902, deflectable post 1904, mount 1914 and hex extension 1912. Itshould be noted that hex extension 1912 also comprises the nipple 1918on the most proximal end. Hex extension 1915 is breakaway component andnipple 1918 allows hex extension 1915 to be secured by a tool uponbreakaway. Where hex extension 1912 meets mount 1914 is a groove 1916.Groove 1916 reduces the diameter of ball rod 1960 such that hexextension 1912 breaks away from mount 1914 when a desired level oftorque is reached during attachment of a vertical rod. The breakawaytorque is determined by the diameter of remaining material and thematerial properties. In a preferred embodiment the breakaway torque isapproximately 30 foot pounds. Thus, hex extension 1912 breaks awayduring implantation and is removed. If mount 1914 needs to be removedfrom a vertical rod, it is necessary to grip another area of the ballrod 1960. Thus, deflectable post 1904 is provided with flats 1917immediately adjacent mount 1914. Flats 1917 allow ball rod 1960 to beengaged by a tool to remove a vertical rod after hex extension 1912 hasbeen removed.

Referring again to FIG. 19A, a cap 1910, in this embodiment is designedto perform multiple functions including securing o-ring 1906 as well assecuring retainer 1902 in cavity 1932 of bone anchor 1920. Cap 1910 isalso larger than cap 1810 of FIG. 18A. In the embodiment of FIG. 19A,cap 1910 has an outer surface 1934, with the below discussedsplines/flutes 1936, adapted for mounting a component, e.g. an offsetconnector. Housing 1930 may in some embodiments be cylindrical aspreviously described. As shown in FIG. 19A, outer surface 1934 ofhousing 1930 is provided with the splines/flutes 1936. Splines/flutes1936 may be engaged by a driver that mates with splines/flutes 1936 forimplanting bone anchor 1920. Cap 1910, by integrating the functions ofthe collar and sleeve, reduces the complexity of the deflection rodassembly 1900 and also increases the strength of the deflection rodassembly 1900 or allows a reduction in size for the same strength. Cap1910 is preferably formed in one piece of titanium or titanium alloy.

As shown in FIG. 19A, cap 1910 comprises a cylindrical shield section1908 connected to a collar section 1909. Cap 1910 is designed to matewith cavity 1932 of housing 1930. Shield section 1908 is threadedadjacent collar section 1909 in order to engage threaded aperture 1932of housing 1930. The distal end of shield section 1908 comprises aflange 1911 for securing ball 1902 within housing 1930. FIG. 19E shows adetailed view of cap 1910 in partial section. As shown in FIG. 19E,flange 1911 at the distal end of cylindrical shield section 1908 has acurved surface having the same radius of curvature as ball 1902. Thecurved surface of flange 1911 in combination with the pocket 1932 inhousing 1930 forms a spherical pocket which traps ball 1902 in a mannerthat allows pivoting and rotation. Inside the central bore of cap 1910is a circumferential groove 1905 designed to hold an o-ring 1906. Groove1905 is cut within the interior of collar section 1909. At the distalend of collar section 1909 are the limit surfaces 1913. Groove 1905 isshaped to support o-ring 1906, reduce wear to o-ring 1906 and reducecreep of the o-ring over time. In this embodiment groove 1905 has arectangular section.

Referring again to FIG. 19A, an o-ring 1906 fits within shield 1908 ofcap 1910 between deflectable post 1904 and cap 1910. In a preferredembodiment o-ring 1906 is a ring with a radiussed square section. O-ring1906 has a round central aperture 1907. Aperture 1907 is slightlysmaller than the diameter of deflectable post 1904 to provide somepreload on assembly. FIG. 19F shows a detailed view of o-ring 1906 inpartial section. FIG. 19E shows o-ring 1906 in position withcircumferential groove 1905 of cap 1910. Note that the external diameterof o-ring 1906 is larger than the interior bore diameter of shield1908/cap 1910. Thus, o-ring 1906 is compressed during assembly andexpands within groove 1905 thereby being retained by groove 1905. Notealso that the interior diameter of aperture 1907 is smaller than thediameter of the bore of shield 1908 so that o-ring 1906 protrudes fromgroove 1905 into the bore around deflectable post 1904 (see FIGS. 19Cand 19D). This o-ring 1906 is formed by a compliant member that iscompressed by deflection of deflectable post 1904. In a preferredembodiment o-ring 1906 is made from polycarbonate urethane (Bionate® 55Dor 80A). But other biocompatible polymers with suitable compliance anddurability may be used. This material is further described in U.S. Pat.No. 5,133,742, issued Jul. 28, 1992, and entitled and U.S. Pat. No.5,229,431, issued Jul. 20, 1993, and entitled “Crack-ResistantPolycarbonate Urethane Polymer Prostheses and the Like,” both of whichare incorporated herein by reference. The o-ring 1906 in FIG. 19F has apreferred shape of flat sides and rounded corners that are transitionedbetween the flat sides. The flat sides allow the o-ring to fit moresecurely in groove 1905 and thus distribute the load placed thereon byball rod 1960 more evenly to reduce wear, creep and deformation of theo-ring. One reason for these advantages is that the flat sides providemore contact surface with the groove 1905 in the cap 1910 and morecontact surface with the ball rod 1960.

FIG. 19A also shows the components of a preferred embodiment of adynamic vertical rod 1950 for use with deflection rod assembly 1900.Dynamic vertical rod 1950 includes a ball 1944 and race 1946. Ball 1944is preferably made of cobalt chrome alloy for better wear. Ball 1944 mayalternatively be made of titanium or titanium alloy with a cobalt chromecoating. Ball 1944 has a central aperture 1945 designed to be secured tomount 1914. Central aperture 1945 is threaded to enable ball 1944 to besecured to the threads of mount 1914. Central aperture 1945 also has afemale hex socket 1947 which may mate with a wrench and by which ball1944 may be tightened to the threaded end of mount 1914. Ball 1944 isreceived in a spherical pocket 1942 in the end of vertical rod 1950.Ball 1944 is secured in spherical pocket 1942 by race 1946. Race 1946 issecured to vertical rod 1950 by, for example, threads and/or laserwelding. When secured, ball 1944 may rotate and pivot in the sphericalpocket 1942. Advantageously, there is no nut extending beyond ball 1944thus reducing the profile of the connection between mount 1914 andvertical rod 1950. To put it another way, the ball 1944 acts as its ownnut to secure ball 1944 to mount 1914. Ball joint 1940 allows greaterrange of motion and reduces torsional stresses on the dynamicstabilization assembly and the bones to which it is attached.

Referring now to FIG. 19B, which shows a perspective view of adeflection rod assembly 1900 assembled with a bone anchor 1920 having abone screw 1922. When assembled, deflectable post 1904 is positionedwithin cap 1910 which is positioned within housing 1930 of bone anchor1920. O-ring 1906 (not seen in this view) is first positioned withinshield 1908 of cap 1910. O-ring 1906 is compressed during introductionto cap 1910 and expands into a groove 1905 within cap 1906 (see FIGS.19C and 19D). A mandrel is used during insertion to prevent damage too-ring 1906. Deflectable post 1904 is then positioned through o-ring1906 and cap 1910 with mount 1914 extending from the proximal end of cap1910. Deflectable post 1904, o-ring 1906 and cap 1910 are thenpositioned within the cavity 1932 of housing 1930. The cap 1910 is thensecured to the threaded proximal end of cavity 1932. Cap 1910 mayalternatively or additionally be laser welded to housing 1930 afterinstallation to secure the components. Cap 1910 secures deflectable post1904 and o-ring 1906 within cavity 1932 of bone anchor 1920 (see FIGS.19C and 19D). Deflectable post 1904 extends out of housing 1930 and cap1910 such that mount 1914 is accessible for connection to a vertical rod1950.

Deflection rod assembly 1900 and bone anchor 1920 are assembled prior toimplantation and then implanted in a bone prior to attachment of adynamic vertical rod or other spinal rod. A special tool may be used toengage cap 1910 during implantation (See FIGS. 20A-20D). Cap 1910 hassurface features 1936 for engagement by a wrench to allow cap 1910 to betightened to housing 1930. For example, cap 1910 may be hexagonal oroctagonal in shape or may have splines and/or flutes and/or otherregistration elements on the surface 1934.

FIG. 19B also shows a perspective view of dynamic vertical rod 1950secured to deflection rod assembly 1900. Dynamic vertical rod 1950 isassembled by placing ball 1944 in pocket 1942 of rod 1950. Race 1946 isthen secured into pocket 1942 by threads and/or laser welding. Onceassembled ball 1944 is free to pivot and rotate with the sphericalpocket of dynamic vertical rod 1950. Central aperture 1945 is accessiblefrom either end of pocket 1942 for attachment of deflectable post 1904.To attach the dynamic deflection rod assembly 1950 to deflectable post1904, ball 1944 is threaded onto the threads of mount 1914 and tightenedinto place with a special tool (see FIGS. 21A-21D). Hex extension 1912breaks away when sufficient torque is applied to lock ball 1944 to mount1914 and hex extension 1912 is then removed.

FIG. 19C shows a sectional view of a deflection rod assembly 1900assembled with a bone anchor 1920 along the axis indicated by line C-Cof FIG. 19B. Retainer 1902 fits into a hemispherical pocket 1939 in thebottom of cavity 1932 of housing 1930. The bottom edge of cap 1910includes a curved flange 1911 which secures ball-shaped retainer 1902within hemispherical pocket 1939 while allowing rotation of ball-shapedretainer 1902. As shown in FIG. 19C, o-ring 1906 occupies the spacebetween deflectable post 1904 and shield 1908 of cap 1910. O-ring 1906is secured within groove 1905 of cap 1910. Cap 1910 thereby secures bothretainer 1902 and o-ring 1906 within housing 1930. O-ring 1906 may becompressed by deflection of deflectable post 1904 towards shield 1908 inany direction. Deflectable post 1904 may pivot about ball-shapedretainer 1902 up to 1 mm in any direction before contacting cap 1910(approximately 3 degrees in a preferred embodiment). Dashed line 1937shows the approximate level of the bone surface when bone anchor 1920,having a bone screw 1922, is implanted. In a preferred embodiment thedistance H representing the height of mount 1914 (and thus dynamicvertical rod 1950) above the bone surface is 16 mm. Also shown in FIG.19C is dynamic vertical rod 1950. Dynamic vertical rod has been securedto deflectable post 1904 by securing ball 1944 to mount 1914.

FIG. 19D illustrates the deflection of deflectable post 1904. Applying aforce to mount 1914 through vertical rod 1950 and ball joint 1940 causesdeflection of deflectable post 1904 of deflection rod assembly 1900.Initially, deflectable post 1904 pivots about a pivot point 1903indicated by an X. Deflectable post 1904 may pivot about pivot point1903 in any direction. Concurrently or alternatively, deflectable post1904 can rotate about the long axis of deflectable post 1904 (which alsopasses through pivot point 1903). In this embodiment, pivot point 1903is located at the center of ball-shaped retainer 1902. As shown in FIG.19D, deflection of deflectable post 1904 compresses the material ofo-ring 1906. O-ring 1906 is compressed into groove 1905. Groove 1905 maybe slightly wider than necessary to accommodate o-ring 1906 in orderthat o-ring 1906 may expand axially while being compressed radially. Theextra space in groove 1905 reduces the possibility that o-ring 1906 willbecome pinched between deflectable post 1904 and the inside of cap 1910.The force required to deflect deflectable post 1904 depends upon thedimensions of deflectable post 1904, o-ring 1906, groove 1905 and shield1908 of cap 1910 as well as the attributes of the material of o-ring1906. The o-ring 1906 exerts a centering force back on deflectable post1904 pushing it back towards a position coaxial with bone anchor 1920.Note that due to ball joint 1940, vertical rod 1950 may also pivotrelative to deflectable post 1904 and rotate relative to deflectablepost 1904 without compressing o-ring 1906.

After further deflection, deflectable post 1904 comes into contact withlimit surface 1913 of collar section 1909 of cap 1910. Limit surface1913 is oriented such that when deflectable post 1904 makes contact withlimit surface 1913, the contact is distributed over an area to reducestress on deflectable post 1904. After deflectable post 1904 comes intocontact with limit surface 1913, further deflection requires deformation(bending) of deflectable post 1904. In a preferred embodiment,deflectable post 1904 is a titanium post 5 mm in diameter. Deflectablepost 1904 is relatively stiff, and the force required to deflectdeflectable post 1904 therefore increases significantly after contact ofdeflectable post 1904 with cap 1910. In a preferred embodiment,deflectable post 1904 may deflect from 0.5 mm to 2 mm in any directionbefore making contact with limit surface 1913. More preferably,deflectable post 1904 may deflect approximately 1 mm before makingcontact with limit surface 1913.

Implantation and Assembly Tools

FIGS. 20A-20D and 21A-21F show various steps in the implantation andconnection of a dynamic stabilization assembly utilizing embodiments ofthe dynamic bone anchor and dynamic vertical rod described herein. Theimplantation and assembly is preferably performed in a minimallyinvasive manner and, thus, tools are provided to facilitate installationand assembly through cannulae. These tools can also be used in openprocedures. One suitable minimally invasive approach to the lumbar spineis the paraspinal intermuscular approach. This approach is described forexample in “The Paraspinal Sacraspinalis-Splitting Approach to theLumber Spine,” by Leon L. Wiltse et al., The Journal of Bone & JointSurgery, Vol. 50-A, No. 5, July 1968, which is incorporated herein byreference. In general the patient is positioned prone. Incisions aremade posterior to the vertebrae to be stabilized. The dorsal fascia isopened and the paraspinal muscle is split to expose the facet joints andlateral processes of the vertebra. Dynamic bone anchors according toembodiments of the present invention and conventional pedicle screws areplaced in the vertebrae as necessary for the selected assembly. Thescrews are placed lateral to the facet joints and angled in towards thevertebral body. The dynamic rods according to embodiments of the presentinvention are then inserted into position adjacent the dynamic boneanchors according to embodiments of the present invention, screws andconventional pedicle screws. The balls of the dynamic rods according toembodiments of the present invention are then secured to the deflectableposts of the dynamic bone anchors according to embodiments of thepresent invention the other end of the dynamic rod is then connected tothe conventional screws with the desired interpediclular distance. Theimplantation of the dynamic bone anchors and connection of the dynamicrods can be facilitated by the implantation tool (FIGS. 20A-20D) andconnection tool (FIGS. 21A-21F) described below.

FIG. 20A shows a perspective view of an implantation tool 2050 for usein implanting a dynamic bone anchor 2000. Dynamic bone anchor 2000 mayfor example be the assembly of deflection rod assembly 1900 and boneanchor 1920 as shown in FIG. 19B. Implantation tool 2050 includes aninner shaft 2060 received within a tubular sleeve 2070. Inner shaft 2060is free to rotate within sleeve 2070. Sleeve 2070 may also be slidtowards the proximal end of inner shaft 2060 by pulling on grip 2074. Acoil spring 2072 is connected between the sleeve 2070 and inner shaft2060 to hold sleeve 2070 in its more distal position relative to shaft2060. The length and diameter of implantation tool 2050 is selected soas to allow use through a cannula in a minimally invasive surgicaltechnique thereby reducing disruption of tissues adjacent theimplantation site, reducing patient recovery and improving surgicaloutcomes.

Referring again to FIG. 20A, shaft 2060 has at a proximal end a quickrelease mount 2062 to which a handle (not shown) may be attached forturning inner shaft 2060. Suitable handles for attachment to shaft 2060include ratcheting handles, torque sensing handles and torque limitinghandles. In alternative embodiments, a handle may be permanentlyconnected to or integrated with the proximal end of shaft 2062. Innershaft has at a distal end a head 2064. Head 2064 includes means forengaging and securing dynamic bone anchor 2000 during implantation as isdescribed below.

As also shown in FIG. 20A, head 2064 can be received over the proximalportion of dynamic bone anchor 2000 with the ball rod 2006 receivedwithin shaft 2060 (see dashed line). In use, dynamic bone anchor 2000 isinserted into the head 2064 of shaft 2060 with the cap 2010 engaged byhead 2064 and the ball rod 2006 secured within head 2064. Dynamic boneanchor 2000 is thus secured to implantation tool 2050. Dynamic boneanchor 2000 will not be released unless and until the surgeon pulls backon grip 2074. Thus, dynamic bone anchor 2000 and implantation tool canbe inserted as one unit through a cannula to the implantation locationin the spine facilitating the positioning and implantation of dynamicbone anchor 2000.

FIG. 20B shows a detailed sectional view of the head 2064 of theimplantation tool 2050 of FIG. 20A engaged with a dynamic bone anchor2000. As shown in FIG. 20B, head 2064 includes a socket 2065 forreceiving and engaging cap 2010 of dynamic bone anchor 2000. Socket 2065is designed to mate with cap 2010 in order to rotate the threaded shank2020 of dynamic bone anchor 2000. Thus, the interior of socket 2065 maybe hexagonal, octagonal or provided with flutes/splines etc., dependingon the particular configuration of the cap 2010. Socket 2065 should beable to apply sufficient torque to cap 2010 to implant the dynamic boneanchor 2000 in a pedicle.

Referring again to FIG. 20B, head 2064 also includes a bore 2065 forreceiving ball rod 2006 of dynamic bone anchor. As shown in FIG. 20B,ball rod 2006 includes a nipple 2018 at the proximal end. A ball 2052 ispositioned within an aperture 2067 which passes from the exterior ofshaft 2060 intersecting bore 2065 adjacent nipple 2018. Ball 2052 isheld by sleeve 2070 in a position in which ball 2052 protrudes into bore2065 so as to trap nipple 2052 within bore 2065. In a preferredembodiment, there are three such balls, however, only one is shown inthis sectional view. Thus, cap 2010 is received in socket 2065 anddynamic bone anchor 2000 is locked to implantation tool 2050 by theinteraction of nipple 2018 and ball(s) 2052.

FIG. 20C shows a detailed sectional view of the head 2064 of theimplantation tool 2050 of FIG. 20A configured to release a dynamic boneanchor 2000. After implantation of dynamic bone anchor 2000 it isnecessary to remove implantation tool 2050. The first step is to slidesleeve 2070 proximally relative to shaft 2060 as shown by arrow A. Thisis achieved by pulling back on grip 2074 against the force of spring2072 (See FIG. 20A). As sleeve 2060 is pulled proximally, ball(s) 2052enters a portion of sleeve 2060 with a larger internal diameter. Ball(s)2052 can move away from engagement with ball rod 2006 as they pass ramp2065 releasing nipple 2018. At this stage both shaft 2060 and sleeve2070 can be pulled together away from dynamic bone anchor 2000.

FIG. 20D shows a transverse view of the lumbar spine illustrating use ofthe implantation tool 2050 of FIG. 20A to implant dynamic bone anchors2000 in the pedicles 2082 of a lumbar vertebra 2084 according to anembodiment of the invention. As shown in FIG. 20D, implantation tool2050 may be used through a cannula 2080 to implant the dynamic boneanchor in a minimally invasive procedure. The cannula 2080 is introducedto the patient to approach the pedicles posteriorly. The pedicle 2082 ofthe vertebra is 2084 is exposed in the conventional fashion. A hole 2086is then drilled through the pedicle 2082 into the vertebral body 2083 ofthe vertebra. Next a dynamic bone anchor 2000 is selected having ofsuitable length, diameter and force/deflection characteristics isselected for implantation. The cap 2010 of the selected dynamic boneanchor 2000 is inserted into the head 2064 of implantation tool 2050 andsecured in place.

Referring now to the left side of FIG. 20D, dynamic bone anchor 2000 andimplantation tool 2050 are inserted as one assembly through cannula 2080to the implantation site. Then dynamic bone anchor is implanted byturning a handle 2088 attached to the quick release on the proximal endof shaft 2060. The dynamic bone anchor 2000 is driven into hole 2086until the housing is at the surface of the vertebra 2084 (see arrow2090). The torque to drive dynamic bone anchor 2000 is provided byhandle 2088 through shaft 2060 to cap 2010 of dynamic bone anchor 2000.

Referring now to the right side of FIG. 20D, when dynamic bone anchor2000 is correctly positioned in pedicle 2082, the physician pulls backon grip 2074 against the force of spring 2072. Sleeve 2070 movesproximally relative to shaft 2060. Shaft 2060 releases the grip ondynamic bone screw 2000 and the both shaft 2060 and sleeve 2070 moveaway from cannula 2080 and out of the patient (see arrow 2092). Dynamicbone anchor 2000 is now correctly implanted and prepared for attachmentto spinal rod and/or other spinal stabilization assembly components.

FIGS. 21A-21D show views of an attachment tool for securing a dynamicvertical rod 2100 to a dynamic bone anchor 2000 according to anembodiment of the invention. FIG. 21A shows a perspective view of anattachment tool 2150 for securing a dynamic vertical rod 2100 to adynamic bone anchor 2000 (shown in FIG. 21C) according to an embodimentof the invention. Dynamic vertical rod 2100 may be, for example, thedynamic vertical rod 1950 of FIG. 19B. Dynamic bone anchor 2000 may be,for example, the assembly of deflection rod assembly 1900 and boneanchor 1920 as shown in FIG. 19B.

Referring first to FIG. 21A, attachment tool 2150 includes an innershaft 2160 received within a tubular sleeve 2170. The length anddiameter of attachment tool 2150 is selected so as to allow use througha cannula in a minimally invasive surgical technique thereby reducingdisruption of tissues adjacent the implantation site, reducing patientrecovery time and improving surgical outcomes. Inner shaft 2160 is freeto rotate and slide within sleeve 2170. Inner shaft 2160 has at aproximal end an attached handle 2162. In alternative embodiments shaft2160 may have a fitting to which a handle might be attached, forexample, ratcheting handles, torque sensing handles and torque limitinghandles. Inner shaft has at a distal end a head 2164 for engaging andsecuring the hex extension of a dynamic vertical rod 2100 (see FIG.21B).

Referring again to FIG. 21A, sleeve 2170 includes a butterfly grip 2174at the proximal end thereof. Sleeve 2170, has at the distal end thereof,means for engaging and securing the female hex socket of a ball of adynamic vertical rod 2100 during connection to a dynamic bone anchor asis described below. In a preferred embodiment head 2164 includes a malehex fitting 2172 with a central aperture 2173. FIG. 21B shows anenlarged view of head 2164 from the distal end of attachment tool 2150.FIG. 21B shows male hex fitting 2172 with central aperture 2173. Throughcentral aperture 2173 is visible female hex socket 2165 of head 2164.Protruding into female hex socket 2165 are two spring tabs 2167.

FIGS. 21C and 21D show detailed sectional views of the distal endattachment tool 2150 in relation to a dynamic vertical rod 2100 anddynamic bone anchor 2000. Referring first to FIG. 21C, which shows adetailed sectional view of the distal end of the attachment tool 2150 ofFIG. 21A, engaged with a dynamic vertical rod 2100 and a dynamic boneanchor 2000. As shown in FIG. 21C, male hex fitting 2172 of head 2164 ofouter sleeve 2170 fits into the female hex socket of ball 2144. At thesame time a hex extension 2115 of ball rod 2006 is received withinfemale hex socket 2165 of inner shaft 2160. When thus engaged, turninghandle 2162 relative to butterfly grip 2174 (See FIG. 21A) can rotateball rod 2006 relative to ball 2144. Attachment tool 2150 is designed toapply sufficient torque to ball rod 2006 relative to ball 2144 to secureball rod 2006 to ball 2144 and breakaway the hex extension 2115 of ballrod 2006. In a preferred embodiment, attachment tool 2150 should be ableto provide greater than 30 foot pounds of torque.

FIG. 21D shows a detailed sectional view of the distal end of theattachment tool 2150 of FIG. 21A after break away of hex extension 2115of ball rod 2006. As shown in FIG. 21D, when ball 2144 has beentightened onto ball rod 2006, tabs 2167 on central aperture 2173 engageeither side of a nipple 2118 of hex extension 2115 to secure hexextension 2115 within female hex socket 2165. Thus, when hex extension2115 beaks away it can be removed from the patient with connection tool2150 as shown.

FIGS. 21E-21H are lateral views of the lumbar spine illustrating stepsof attaching a dynamic vertical rod 2100 to a dynamic bone anchor 2000utilizing the attachment tool of FIG. 21A according to an embodiment ofthe invention. As shown in FIG. 21E, the dynamic vertical rod 2100 isimplanted after the dynamic bone anchor 2000 and a polyaxial screw 2140have already been implanted. Dynamic vertical rod 2100 is implanted in acranially direction—preferably in a minimally invasive manner untildynamic vertical rod 2100 is positioned adjacent dynamic bone anchor2000 and polyaxial screw 2140. The hex extension 2115 of dynamic boneanchor 2000 is then fed through ball 2144 of dynamic vertical rod 2100as shown.

Next, as shown in FIG. 21F, connection tool 2150 is inserted through acannula 2080 to engage ball 2144 and hex extension 2115. Ball 2144 isthen turned relative to hex extension 2115 until it is fully secured toball rod 2006. When ball 2144 is fully secured to ball rod 2006, furthertorque is applied until hex extension 2115 (not shown) is sheared off.In a preferred embodiment, this requires 30 foot pounds of torque and issufficient to lock ball 2144 to ball rod 2006. Next, as shown in FIG.21G, connection tool 2150 can be removed from cannula 2080. Aspreviously described, hex extension 2115 (not shown) is retained insideattachment tool 2150 for easy removal from the patient. As shown in FIG.21H a conventional tool 2184 is then inserted through cannula 2180 tooperate polyaxial screw 2140 to secure the other end of dynamic verticalrod 2100.

Preserving Anatomically Correct Motion of the Spine

FIG. 22A is a lateral view of the lumbar spine illustrating the naturalkinematics of the spine during extension and flexion. A superiorvertebra 2200 (for example L4) is shown relative to an inferior vertebra2210 (for example L5). The primary load bearing structures are thevertebral bodies 2202 and 2102. Between the vertebral bodies lies anintervertebral disc 2220. Dorsal of the spinal bodies lie the pedicles2204, 2214, facets 2206, 2216 and spinous processes 2208, 2218. Betweenthe spinous process is a ligamentous band called the interspinousligament 2222. In the healthy lumbar spines significant extension andflexion of the spine is possible in the lumbar region—approximating 35degrees of total flexion over the entire lumbar region. As the spineflexes and extends the vertebrae move relative to one another whilemaintaining alignment of the vertebral bodies to support the weight ofthe upper body.

Between extension and flexion, the superior vertebra 2200 may movethrough an angle or range of about 15 degrees with respect to theinferior vertebra 2210. In the healthy spine the natural center ofrotation 2224 for this rotation is located within the intervertebraldisc 2220. Rotation about the natural center of rotation 2224 causeselongation of the interspinous ligament 2222 and slight separation ofthe facets 2206, 2216. However, this rotary motion does not occur alone.The healthy spine exhibits a phenomenon called coupling in whichrotation or translation about or along one axis or plane is consistentlyassociated with another motion about or along a second axis or plane.The dashed line 2200 a shows the position of the superior vertebraduring flexion. As can be seen, during flexion, not only does thesuperior vertebra 2200 rotate about the natural center of rotation 2224,but it also translates cranially and dorsally. As a consequence, normalflexion also induces up to approximately an 8 mm increase in thedistance between the pedicles 2204, 2214 from a combination of elevationand forward translation. This is enabled by elongation of theinterspinous band and facet separation. Similarly, lateral bending ofthe spine is coupled with relative axial rotation of the vertebrae.

With age, the vertebral bodies of the spine and intervertebral discs candegenerate. This spinal degeneration reduces the load-bearing ability ofthe spine, causes pain, reduces range of motion and can inducecompensatory bone growth. The bone growth can lead to further reductionin range of motion and spinal stenosis in which the bone compressesblood vessels and nerves passing along the spine leading to inflammationand more pain. A number of spinal prosthesis have been proposed tomaintain or restore the load-bearing capability of the spine, reducediscogenic instability, provide pain relief after discectomy, to top offdegenerative discs above or below vertebral fusion, and/or to supportdegenerative discs without fusion. The basic objectives of suchprostheses are load sharing and stabilization of the spine to remediatethe problems identified above and reduce pain. Unfortunately, the spineis a very complex structure and it is very difficult to provide aprosthesis for load sharing and stabilization that does not also changethe natural kinematics of the spine causing additional artifacts,instabilities and as a result further degeneration of the spine.

FIG. 22B is a lateral view of the lumbar spine illustrating thekinematic constraints placed on the spine by a rigid spinal rod systemduring extension and flexion during extension and flexion. FIG. 22Bshows a pedicle screw 2230 implanted in the superior vertebra 2200 and apedicle screw 2232 implanted in the inferior vertebra 2210. The pediclescrews are connected by a rigid vertical rod 2234. The vertical rod 2234and screws form a theoretically rigid system. The vertical rod thustransmits some of the load from the superior vertebra 2200 to theinferior vertebra 2210 thereby reducing the load on the vertebral bodies2202, 2212 and the intervertebral disc 2220.

However, an artifact of a rigid prosthesis as shown in FIG. 22B, is thatthe relative rotation of the vertebra is constrained and theinterpedicular distance is fixed. During flexion of the spine, somerotation is permitted by flexing of the vertical rod 2234 and theconnections between the vertical rod 2234 and the pedicle screws 2230and 2232. However, because the interpedicular distance remainsessentially fixed, no elongation of the interspinous ligament 2222 ispossible and the center of rotation 2236 is moved significantly dorsallyof the natural center of rotation to the dorsal edge of theintervertebral disc or even further. The dashed lines 2200 b show therelative movement of the superior vertebra 2200. Moreover, not only isfacet separation prevented but the flexure about the new center ofrotation can actually push the facets together increasing loading of thefacet joints 2206, 2216. The prosthesis also interferes with the naturalcoupling of the spine by precluding and/or limiting the translation ofthe superior vertebra which is associated with rotation in naturalflexion. Additionally, the flexing of the vertical rod placessignificant strain upon the pedicle screws and the interface between thepedicle screws 2230, 2232 and the bone which can lead either to devicefailure, backing out of the screws or damage to the pedicles.Furthermore, constraining motion at one segment of the spine is thoughtto create additional stress at adjacent segments and might thereforeaccelerate degeneration at those spinal segments (adjacent-leveldisease).

In order to overcome the problems caused by a rigid spinal prosthesis, adynamic spine stabilization prosthesis attempts to preserve anatomicalspinal motion and motion quality. An ideal prosthesis should be able tomaintain intersegmental stability and permit appropriate motion at aspinal segment, e.g. ˜15 degrees of flexion/extension, ˜2 degrees ofaxial rotation, ˜6 degrees lateral bending as well as relativetranslation of the vertebrae ˜2 mm of left-right yaw, ˜2 mm of elevation(separation), and/or ˜2 mm of dorsal-ventral shift. The ideal prosthesisshould also allow complex combinations of these motions and permit thecoupling exhibited in the anatomical spine. The prosthesis should beable to preserve these motions required for normal spinal function whileproviding load sharing without abnormal load distribution, and spinalsegment stabilization including limiting motion beyond anatomicallydesirable limits.

FIGS. 22C and 22D show the kinematic modes of a dynamic spinestabilization prosthesis utilizing a dynamic bone anchor and dynamicvertical rod in accordance with embodiments of the invention. FIGS. 22Cand 22D show kinematic modes of a dynamic bone anchor 2240 inconjunction with a dynamic vertical rod 2250. Dynamic bone anchor 2240includes a ball rod 2242 that pivots about ball 2244 at the distal endrelative to threaded anchor 2246. Ball rod 2242 is connected at itsproximal end to ball 2254 of dynamic vertical rod 2250. Deflection ofball rod 2242 relative to threaded anchor 2246 of the dynamic boneanchor 2240 is controlled by compression of compliant ring 2245 andlimited by hard contact surfaces at the proximal end of the cap 2248.The three links—vertical rod 2250, ball rod 2242, and threaded anchor2246—and two ball joints 2244, 2254 are connected in series and, thus,the movements of the linkages can be combined to provide a complex rangeof kinematic modes.

FIG. 22C shows the kinematic modes of ball rod 2242 relative to dynamicvertical rod 2250 assuming no motion internal to dynamic bone anchor2240. As shown in FIG. 22C, ball rod 2242 pivots and rotates about ball2254 of dynamic vertical rod 2250. Ball rod 2242 (and threaded anchor2246) can pivot 15 degrees in any direction from perpendicular relativeto dynamic vertical rod 2250 as shown by arrow 2260 for a total range ofmotion of 30 degrees. Ball rod 2242 (and threaded anchor 2246) can alsorotate 360 degrees relative to dynamic vertical rod 2250 as shown byarrow 2262.

FIG. 22D shows the kinematic modes of threaded anchor 2246 relative toball rod 2242 based solely on internal motion within dynamic bone anchor2240. As shown in FIG. 22D, threaded anchor 2240 pivots and rotatesabout ball 2244 of ball rod 2242. Threaded anchor 2246 can pivot 3degrees in any direction from perpendicular relative to ball rod 2242 asshown by arrow 2264 for a total range of motion of 6 degrees. Dynamicvertical rod can also rotate 360 degrees relative to ball rod 2242 asshown by arrow 2266.

The kinematics of the ball rod 2242 relative to dynamic vertical rod2250 and the threaded anchor 2246 relative ball rod 2242 combine togenerate more complex kinematics than would be available with eithercomponent alone. The compound kinematics more closely approximate thenatural kinematics of the spine. FIGS. 22E and 22F illustrate thecompound kinematics of a dynamic spinal stabilization prosthesisincorporating a dynamic bone anchor 2240 and dynamic vertical rod 2250.FIG. 22E is a simplified illustration of the kinematics of a dynamicspine stabilization prosthesis showing the movement of dynamic boneanchor 2240 relative to a fixed bone anchor 2241. FIG. 22F is a lateralview of the spine illustrating the kinematics of a spinal segmentsupported by the dynamic spine stabilization prosthesis of FIG. 22E.

As shown in FIGS. 22E and 22F, a dynamic spinal prosthesis incorporatingboth the dynamic bone anchor 2240 and dynamic vertical rod 2250 allowsnot only rotary motion (arrow 2270) but also coupled translation (arrow2272) of a dynamic bone anchor 2240 relative to a bone anchor 2241.Furthermore the center of rotation 2274 is maintained at an anatomicallydesirable position in the intervertebral disc (See FIG. 22F).Maintenance of the natural center of rotation helps prevent unevenloading of the vertebral bodies 2202, 2212. The pivoting motion andtranslation are coupled and compliantly modulated by compression of thecompliant member of the dynamic bone anchor (see FIG. 22D). Moreover,the prosthesis also limits the availability movement by, for example,contact between the ball rod 2242 and the cap 2248 thus providingsegmental stability. The kinematics of threaded anchor 2246 of dynamicbone anchor 2240 thus closely approximate the natural kinematics of thevertebra shown in FIG. 22A. Consequently, a dynamic spinal stabilizationprosthesis incorporating both dynamic vertical rod 2250 and dynamic boneanchor 2240 can stabilize the spine and provide load sharing whilemaintaining the center of rotation of implant vertebra within theintervertebral disc 2220 close to the natural center of rotation (seeFIG. 22A) of the spine preserving natural range of motion. Thekinematics of the prosthesis by allowing translation of vertebra 2200relative to vertebra 2210 also serve to preserve facet separation duringflexion seen in the natural spine. By allowing more natural kinematics,stain on the components and the bone interface is reduced leading toenhanced durability, safety and efficacy.

The rotation of the ball rod 2242 relative to the dynamic vertical rod2250 and threaded anchor 2246 relative to the ball rod 2242 (see FIGS.22C, 22D) also permit kinematics impossible with rigid pedicle screwsystems. For example, lateral bending of the spine couples with relativerotation of the vertebrae. In the rigid spinal implant of FIG. 22B,there is no provision for such rotation which would therefore resolve asstrain upon the components and component/bone interface. However, in adynamic spinal prosthesis incorporating both dynamic vertical rod 2250and dynamic bone anchor 2240, rotation is provided about ball 2244 andball 2254, thus allowing both changes in the side-to-side intervertebraldistance and coupled axial rotation of the vertebrae closelyapproximating the natural kinematics of the spine. Dynamic stabilizationassemblies incorporating embodiments of the present invention can alsosupport complex combinations of natural movements and the coupledrotations and translations of the spine, for example, lateral bendingwith twisting, lateral bending with flexion. Thus, anatomically correctmotion of the spine is stabilized and preserved.

The close approximation of the kinematics of the dynamic spinalprosthesis and the natural kinematics of the spine results in reducedstresses at the implant/bone interface and, by using a natural center ofrotation, allows even stress distribution across the vertebral bodiesand intervertebral disc. The prosthesis has a decreased stiffness andincreased range of motion compared to conventional rigid vertical rodsystems supporting the implant segment while reducing stresses onadjacent segments. However, the dynamic spine stabilization prosthesis,with the compliant element located in-line within the dynamic boneanchor, is more robust than flexible rod systems. The degree ofcompliance in the dynamic bone anchor can also be tailored for theindividual based upon load and anatomy. The result is anatomical loaddisplacement curves, stabilization and preservation of anatomicallycorrect motion and a robust surgical remediation of spinal degeneration.

Deflection Rod/Loading Rod Materials

Movement of the deflectable post relative to the bone anchor providesload sharing and dynamic stabilization properties to the dynamicstabilization assembly. As described above, deflection of thedeflectable post deforms the material of the sleeve. The characteristicsof the material of the sleeve in combination with the dimensions of thecomponents of the deflection rod assembly affect the force-deflectioncurve of the deflection rod. The dimensions and materials may beselected to achieve the desired force-deflection characteristics.

By changing the dimensions of the deflectable post, sleeve and theshield, the deflection characteristics of the deflection rod assemblycan be changed. The stiffness of components of the deflection rodassembly can be, for example, increased by increasing the diameter ofthe deflectable post and/or by decreasing the diameter of the innersurface of the shield. Additionally, decreasing the diameter of thedeflectable post will decrease the stiffness of the deflection rodassembly while decreasing the diameter of the post and/or by increasingthe diameter of the inner surface of the shield will decrease thestiffness of the deflection rod. Alternatively and/or additionally,changing the materials which comprise the components of the deflectionrod assembly can also affect the stiffness and range of motion of thedeflection rod. For example, making the sleeve out of stiffer and/orharder material reduces deflection of the deflectable post.

The deflectable post, bone anchor and vertical rods are preferably madeof biocompatible implantable metals. The deflectable post can, forexample, be made of titanium, titanium alloy, cobalt chrome, a shapememory metal, for example, Nitinol (NiTi) or stainless steel. Inpreferred embodiments, the deflectable post is made of cobalt chrome. Inpreferred embodiments, the bone anchor and vertical rods are made oftitanium alloy; however, other materials, for example, stainless steelmay be used instead of or in addition to the titanium components.Furthermore, the ball of the dynamic vertical rod is preferably made ofcobalt chrome for good wear characteristics.

The material of the sleeve/compliant member/or-ring is a biocompatibleand implantable polymer having the desired deformation characteristics.The material of the sleeve should also be able to maintain the desireddeformation characteristics. Thus the material of the sleeve ispreferably durable, resistant to oxidation and dimensionally stableunder the conditions found in the human body. The sleeve may, forexample be made from a polycarbonate urethane (PCU) such as Bionate®. Ifthe sleeve is comprised of Bionate®, a polycarbonate urethane or otherhydrophilic polymer, the sleeve can also act as a fluid-lubricatedbearing for rotation of the deflectable post relative to thelongitudinal axis of the deflectable post.

Suitable materials for the sleeve include polyurethanes includingpolycarbonate-urethanes (PCU). Suitable PCUs are available under thetrade name BIONATE® from the Polymer Technology Group—DSM PTG, Inc.(Berkeley, Calif.). Bionate® PCU has good biocompatibility and has beenFDA approved for long-term implantation. Bionate® PCU has good oxidativestability, biocompatibility, mechanical strength and abrasion resistanceand suitable physical properties including load bearing, dimensionalstability and resistance to environmental stress cracking. Bionate® PCUis also available in five hardness grades 80A, 90A, 55D, 65D and 75D—thedifferent hardness grades imparting different deflection characteristicsto components incorporating them. In a preferred embodiment, the sleeveis made of grade 80A Bionate® PCU which is 2 mm thick when uncompressedand may be compressed to about 1 mm in thickness by deflection of thepost.

The sleeve can be formed by extrusion, injection, compression moldingand/or machining techniques, as would be appreciated by those skilled inthe art. In some embodiments, the sleeve is formed separately. Forexample, a sleeve may be cut or machined from a biocompatible polymerand then assembled with the deflectable post and sleeve such as by beingpress fit into the shield. Alternatively or additionally, abiocompatible adhesive may be used to bond the sleeve to the shieldand/or post. In alternative embodiments, the sleeve may be formed inplace by positioning the post and inside the shield and then filling thespace between the deflectable post and the shield with liquid polymer(polymer reagents) and allowing the polymer to solidify.

A one piece PCU sleeve/compliant member/o-ring may be produced, forexample by multi-shot or insert injection molding yielding densitygradients which can be used to control the force/deflection responsecurve of the deflection rod. Voids, gaps or other structural featuresmay also be provided to modify the compliance of the sleeve andconsequently the force/deflection response curve of the deflection rod.The density gradients may be patterned in order to control the responsecurve of the deflection rod. The density gradients need not besymmetric. Directional variations in the density gradients may be usedto create a deflection rod assembly having different force/deflectionresponses in different directions.

The sleeve may also include polymer regions having different properties.For example, the sleeve can include concentric rings of one or morepolymers with each ring having a different hardness of stiffness ordurometer. For example, each successive ring from the center outward canhave a higher hardness or stiffness or durometer so that as the post isdeflected outwardly from a position that is collinear with thelongitudinal axis the sleeve provides increased resistance to furtherdeflection. The sleeve may also be designed to provide different forcedeflection characteristics in different directions. The deflectable postcould also be designed so that less resistance occurs with increaseddeflection of the post.

Other polymers or thermoplastics may be used to make the sleeveincluding, but not limited to, polyether-etherketone (PEEK),polyphenylsolfone (Radel®), or polyetherimide resin (Ultem®). Otherpolymers that may be suitable for use in some embodiments, for exampleother grades of PEEK, for example 30% glass-filled or 30% carbon filled,provided such materials are cleared for use in implantable devices bythe FDA, or other regulatory body. Glass-filled PEEK is known to beideal for improved strength, stiffness, or stability while carbon filledPEEK is known to enhance the compressive strength and stiffness of PEEKand lower its expansion rate.

Still other suitable biocompatible thermoplastic or thermoplasticpolycondensate materials may be suitable, including materials that havegood memory, are flexible, and/or deflectable have very low moistureabsorption, and good wear and/or abrasion resistance, can be usedwithout departing from the scope of the invention. These includepolyetherketoneketone (PEKK), polyetherketone (PEK),polyetherketoneetherketoneketone (PEKEKK), andpolyetheretherketoneketone (PEEKK), and generally apolyaryletheretherketone. Further, other polyketones can be used as wellas other thermoplastics.

PCU materials suitable for implantation are described in U.S. Pat. No.5,133,742 titled “Crack-Resistant Polycarbonate Urethane PolymerProstheses” and U.S. Pat. No. 5,299,431 titled “Crack-ResistantPolycarbonate Urethane Polymer Prostheses And The Like”, both of whichpatents are incorporated herein by reference. Other polymers that can beused in the sleeve are disclosed in the following documents, all ofwhich are incorporated herein by reference. These documents include: PCTPublication WO 02/02158 A1, dated Jan. 10, 2002 and entitledBio-Compatible Polymeric Materials; PCT Publication WO 02/00275 A1,dated Jan. 3, 2002 and entitled Bio-Compatible Polymeric Materials; andPCT Publication WO 02/00270 A1, dated Jan. 3, 2002 and entitledBio-Compatible Polymeric Materials.

The materials of the sleeve may thus be selected to create a deflectionrod assembly having stiffness/deflection characteristics suitable forthe needs of a patient. By selecting appropriate materials of thesleeve, the deflection characteristics of the deflection rod assemblycan be configured to approach the natural dynamic motion of the spine ofa particular patient, while giving dynamic support to the spine in thatregion. It is contemplated, for example, that the deflection rodassembly can be made in stiffness that can replicate a 70% range ofmotion and flexibility of the natural intact spine, a 50% range ofmotion and flexibility of the natural intact spine and a 30% range ofmotion and flexibility of the natural intact spine.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many embodiments were chosenand described in order to best explain the principles of the inventionand its practical application, thereby enabling others skilled in theart to understand the invention for various embodiments and with variousmodifications that are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claims andtheir equivalents.

1. A medical device comprising: a bone screw having a longitudinal axisand a distal end adapted to engage a bone; a head at a proximal end ofsaid bone screw the head including a pocket; a tubular cap having a boreextending completely therethrough along a bore axis, wherein the tubularcap includes a fastener for securing the tubular cap to the head andwherein the tubular cap is fastened to the head such that the bore axisis aligned with the longitudinal axis of the bone screw; a deflectablepost having a proximal end extending from the bore of said tubular capcoaxial to the longitudinal axis of the bone screw when the tubular capis fastened to the head, and a distal end terminating in a retainerreceived in the pocket of the head, and a post axis extending betweenthe proximal end and the distal end and said tubular cap for securingthe retainer in the pocket of the head; and a compliant ring positionedwithin the bore in contact with the deflectable post and the tubular capwhereby the deflectable post can pivot relative to the bone screw andthe post axis is compliantly held in alignment with the longitudinalaxis of the bone screw by the compliant ring.
 2. The medical device ofclaim 1, wherein: the bore has a circumferential groove therein; and thecompliant ring is retained in said circumferential groove.
 3. Themedical device of claim 2, wherein the compliant ring has an externaldiameter larger than the bore diameter and internal diameter less thanthe bore diameter.
 4. The medical device of claim 3 wherein thedeflectable post has a post diameter approximately equal to the internaldiameter of the compliant ring.
 5. The medical device of claim 1,wherein said tubular cap comprises a limit surface positioned to contactthe deflectable post after a predetermined amount of deflection of thepost axis away from alignment with the longitudinal axis of the bonescrew.
 6. The medical device of claim 1, wherein: said tubular capcomprises a limit surface positioned to contact the deflectable postafter a predetermined amount of deflection of the post axis away fromalignment with the longitudinal axis of the bone screw; and thecompliant ring is shaped and positioned to prevent the compliant ringfrom being pinched between the deflectable post and the limit surface.7. The medical device of claim 1, wherein the compliant ring has acircular radial section.
 8. The medical device of claim 1, wherein thecompliant ring has a substantially square radial section.
 9. The medicaldevice of claim 1, wherein the tubular cap has a plurality of externalsurface features adapted to provide for engagement of the tubular cap byat least one of an implantation tool and a connector.
 10. A spinalimplant comprising: an elongate bone anchor having a longitudinal axisextending between a threaded distal end and a proximal end; a posthaving a distal end and a proximal end and a post axis extendingtherebetween; a joint which secures the distal end of the post to theproximal end of the elongate bone anchor such that the post can pivotrelative to the elongate bone anchor; a tubular cap having a boreextending completely therethrough along a bore axis, the tubular capextending over a distal portion of the post, wherein the tubular cap hasa fastener which secures the tubular cap to the proximal end of theelongate bone anchor such that the bore axis is aligned with thelongitudinal axis of the elongate bone anchor; and a compliant ringdisposed in the bore in contact with the post and the tubular capwhereby the compliant ring biases the post axis into alignment with thelongitudinal axis of the elongate bone anchor.
 11. The spinal implant ofclaim 10, wherein: the bore has a circumferential groove therein; andthe compliant ring is retained in said circumferential groove.
 12. Thespinal implant of claim 10, wherein the bore has a bore diameter and thecompliant ring has an external diameter larger than the bore diameterand internal diameter less than the bore diameter.
 13. The spinalimplant of claim 10, wherein the post has a post diameter approximatelyequal to an internal diameter of the compliant ring.
 14. The spinalimplant of claim 10, wherein: said tubular cap comprises a limit surfacepositioned to contact the post after a predetermined amount ofdeflection of the post axis away from alignment with the longitudinalaxis of the elongate bone anchor.
 15. The spinal implant of claim 10,wherein the compliant ring has a circular radial section.
 16. Themedical device of claim 10, wherein the compliant ring has asubstantially square radial section.
 17. The spinal implant of claim 10,wherein the compliant ring comprises a hydrophilic polymer.
 18. Thespinal implant of claim 10 wherein the tubular cap has a plurality ofexternal surface features adapted to engage at least one of animplantation tool and a connector.
 19. A pedicle screw comprising: abody having a longitudinal axis, the body including a threaded cylinderadapted to engage a pedicle of a patient; a housing formed at an end ofthe body opposite the threaded cylinder, the housing including a pockettherein; a post having a distal end with a retainer formed thereon and aproximal end, with a post axis extending between the proximal end andthe distal end of the post; a tubular cap having a bore extendingcompletely therethrough, the tubular cap extending over a portion of thepost, wherein the tubular cap is fastened to the housing so as to trapthe retainer formed on the distal end of the post in the pocket, therebyforming a joint which allows the post axis to pivot relative to thelongitudinal axis of the body of the pedicle screw; and a compliant ringdisposed in the bore, the compliant ring being in contact with the postand the tubular cap whereby the compliant ring biases the post axis intoalignment with the longitudinal axis of the body of the pedicle screw.20. The pedicle screw of claim 19, wherein: the bore has acircumferential groove therein; and the compliant ring is retained insaid circumferential groove.
 21. The spinal implant of claim 20,wherein: the bore has a bore diameter; the compliant ring has anexternal diameter larger than the bore diameter; the compliant ring hasan internal diameter less than the bore diameter; and the post has apost diameter approximately equal to the internal diameter of thecompliant ring.
 22. The pedicle screw of claim 19, wherein said tubularcap comprises a limit surface positioned to contact the post after apredetermined amount of deflection of the post axis away from alignmentwith the longitudinal axis of the body of the pedicle screw.
 23. Thepedicle screw of claim 19 wherein the cap has a plurality of externalsurface features adapted to engage at least one of an implantation tooland a connector.
 24. The medical device of claim 19, wherein thecompliant ring has a substantially square radial section.